CHAPTER XXI.

THE SKELETON OF THE DOG[139] (_Canis familiaris_).

I. EXOSKELETON.

The exoskeleton of the dog includes three sets of structures: 1. hairs, 2. claws, 3. teeth. =Hairs= and =claws= are epidermal exoskeletal structures, while =teeth= are partly of dermal, and partly of epidermal origin.

1. =Hairs= are delicate epidermal structures which grow imbedded in little pits or follicles in the dermis. Specially large hairs forming the =vibrissae= or =whiskers= grow attached to the upper lip.

2. =Claws= are horny epidermal sheaths, one of which fits on to the pointed distal phalanx of each digit. They are sharply curved structures, and being in the dog non-retractile, their points are commonly much blunted by friction with the ground. The claws of the pollex, and of the hallux when it is present, however do not meet the ground, and therefore remain comparatively sharp.

3. =Teeth=[140]. Although as regards their mode of origin, teeth are purely exoskeletal or tegumentary structures, they become so intimately connected with the skull that they appear to belong to the endoskeleton.

Each tooth, as has been already described, consists of three distinct tissues, dentine and cement of dermal origin, and enamel of epidermal origin.

The teeth of the dog (fig. 68) form a regular series arranged along the margins of both upper and lower jaws, and imbedded in pits or =alveoli= of the maxillae, premaxillae, and mandibles. They are all fixed in the bone by tapering roots, and none of them grow from persistent pulps.

They are divisible into four distinct groups, the =incisors=, =canines=, =premolars= and =molars=. There are three incisors, one canine and four premolars on each side of each jaw. But while there are three molars on each side of the lower jaw, the last is wanting in the upper jaw. The dentition of the dog may then be represented by the formula

_i_ 3/3 _c_ 1/1 _pm_ 4/4 _m_ 2/3 × 2 = 42.

In each jaw there is one large specially modified tooth called the =carnassial=, the teeth in front of this are more or less pointed and compressed, while those behind it are more or less flattened and tuberculated.

=Teeth of the upper jaw.=

The first and second =incisors= are small teeth with long conical roots and somewhat chisel-shaped crowns. Surrounding the base of the crown there is a rather prominent ridge, terminated laterally by a pair of small cusps. This ridge, the =cingulum=, serves to protect the edge of the gums from injury by the hard parts of food. The third incisor is a good deal like the others but larger, and has the cingulum well developed though not terminated by lateral cusps. All the incisors are borne by the premaxillae, the remaining teeth by the maxillae.

The =canine= is a large pointed tooth, slightly recurved and with a long tapering root.

The =premolars= are four in number, and in all the cingulum is fairly well seen. The first is a very small tooth with a single tapering root, the second and third are larger and have two roots, while the fourth, the =carnassial=, is much the largest and has three roots. Each of the second, third and fourth premolars has a stout blade, the middle portion of which is drawn out into a prominent cone; the posterior part of the fourth premolar forms a compressed ridge, and at the antero-internal edge of the tooth there is a small inner tubercle.

The two =molar= teeth are of very unequal size. The first, which has two anterior roots and one posterior, is wider than it is long, its outer portion being produced into two prominent cusps, while its inner portion is depressed. The second molar is a small tooth resembling the first in its general appearance, but with much smaller outer cusps.

=Teeth of the lower jaw.=

The three =incisors= of the lower jaw have much the same character as the first two of the upper jaw; while the =canine= is identical in character with that of the upper jaw.

The four =premolars= gradually increase in size from the first to the last, but none are very large. The first premolar is a single-rooted tooth resembling that of the upper jaw; the second, third and fourth are two-rooted, like the second and third of the upper jaw, which they closely resemble in other respects.

The first =molar= forms the =carnassial= (fig. 84, V), and with the exception of the canine, is much the largest tooth of the lower jaw; it is a two-rooted tooth, with a long compressed bilobed blade, and a posterior tuberculated talon or heel. The second molar is much smaller, though likewise two-rooted, while the third molar is very small and has only a single root. All the teeth except the molars are preceded in the young animal by temporary =milk teeth=. These milk teeth, though smaller, are very similar to the permanent teeth by which they are ultimately replaced.

II. ENDOSKELETON.

1. THE AXIAL SKELETON.

This includes the vertebral column, the skull, and the ribs and sternum.

A. THE VERTEBRAL COLUMN.

This consists of a series of about forty vertebrae arranged in succession so that their centra form a continuous rod, and their neural arches a continuous tube, surrounding a cavity, the =neural canal=.

The vertebrae may be readily divided into five groups:--

1. The =cervical= or neck vertebrae.

2. The =thoracic= or chest vertebrae which bear ribs.

3. The =lumbar= vertebrae which are large and ribless.

4. The =sacral= vertebrae which are fused with one another and united with the pelvis.

5. The =caudal= or tail vertebrae which are small.

Except in the sacral region the vertebrae are movably articulated to one another, while their centra are separated from one another by cartilaginous =intervertebral discs=.

GENERAL CHARACTERS OF A VERTEBRA.

Take as a type the =fourth lumbar vertebra=. It may be compared to a short tube whose inner surface is smooth and regular, and whose outer surface is thickened and drawn out in a variety of ways. The basal part of the vertebra is the =centrum= or body which forms the thickened floor of the neural canal. Its two ends are slightly convex and are formed by the =epiphyses=, two thin plates of bone which are at first altogether distinct from the main part of the centrum, but fuse with it as the animal grows older; its sides are drawn out into a pair of strong =transverse processes=, which project forwards, outwards, and slightly downwards. The =neural arch= forms the sides and roof of the neural canal, and at each end just above the centrum bears a pair of =intervertebral notches= for the passage of the spinal nerves, the posterior notches being considerably deeper than the anterior. The neural arch is drawn out into a series of processes. Arising from the centre of the dorsal surface is a prominent median =neural spine= or =spinous process=, which projects upwards and slightly forwards; its anterior edge is vertical, while its posterior edge slopes gradually. At the two ends of the neural arch arise the two pairs of =zygapophyses= or articulating surfaces, which interlock with those of the adjacent vertebrae. The anterior or =prezygapophyses= look inwards, and are large and concave; they are borne upon a pair of large blunt outgrowths of the neural arch, the =metapophyses=. The posterior or =postzygapophyses= are slightly convex and look outwards and downwards; they are borne upon backwardly projecting outgrowths of the neural arch. Lastly there are a pair of minute projections arising from the posterior end of the neural arch, below the postzygapophyses. These are the =anapophyses=. In young individuals the development of all the processes of the various vertebrae is less marked, and the epiphyses are obviously distinct.

THE CERVICAL VERTEBRAE.

These are seven in number, as in almost all mammals. They are characterised by the fact that they have small ribs fused with them, forming transverse processes perforated by canals through which the vertebral arteries run.

The first, or =atlas= vertebra (fig. 69, A), differs much from all the others; it is drawn out into a pair of wide wing-like transverse processes (fig. 69, A, 1), and forms a ring surrounding a large cavity. This cavity is during life divided into two parts by a transverse ligament; the upper cavity is the true neural canal, while the lower lodges the =odontoid process= of the second vertebra, which is the detached centrum of the atlas. The neural arch is broad and regular; it has no spinous process, and is perforated in front by a pair of foramina for the passage of the first spinal nerves. The mid-ventral portion of the atlas is rather thick, and bears a minute backwardly-projecting hypapophysis. The bases of the broad transverse processes are perforated by the =vertebrarterial canals= (fig. 69, A, 2). The atlas bears at each end a pair of large articulating surfaces; those at the anterior end articulate with the condyles of the skull, and are very deeply concave; those at the posterior end for articulation with the axis, are nearly as large, but are flattened. The atlas ossifies from three centres, one forming the mid-ventral portion, the others the two halves of the remainder.

The second, or =axis= vertebra (fig. 69, B), also differs much from the other cervicals. The long and broad centrum has a very flat dorsal surface, and is produced in front into the conical =odontoid process= (fig. 69, B, 5), and bears a pair of very large convex outwardly directed surfaces for articulation with the atlas. At its posterior end it is drawn out into a pair of small backwardly-directed spines, the transverse processes; these are perforated at their bases by the vertebrarterial canals. The neural arch is deeply notched in front and behind for the passage of the spinal nerves, and is drawn out above into a very long compressed neural spine (fig. 69, B, 4), which projects a long way forwards, and behind becomes bifid and thickened, bearing a pair of flat downwardly directed postzygapophyses. In the young animal the odontoid process is readily seen to ossify from a centre anterior to that forming the anterior epiphysis of the axis.

The remaining five cervical vertebrae, the third to the seventh inclusive, have rather flattened wide centra, obliquely truncated at either end. The neural spine progressively increases in size as the vertebrae are followed back. The transverse processes vary considerably; those of the third are divided into a thicker backwardly-, and a more slender forwardly-projecting portion; those of the fourth and fifth mainly extend downwards, and that of the sixth is divided into a horizontal portion and a downwardly-projecting =inferior lamella=. All the cervical vertebrae except the seventh have the bases of the transverse processes perforated by the vertebrarterial canals. The prezygapophyses in each case look upwards and slightly inwards, while the postzygapophyses look downwards and slightly outwards.

THE THORACIC VERTEBRAE.

The =thoracic vertebrae= are twelve or thirteen in number, and all bear movably articulated ribs. As a group they are characterised by their comparative shortness, and in the case of the first eight or nine by the great length of the backwardly-sloping neural spine. The posterior thoracic vertebrae approach in character the succeeding lumbar vertebrae.

As type of the anterior thoracic vertebrae, take any one between the second and sixth inclusive. The centrum is short, and has its terminations vertically truncated. At the top of the centrum, at both anterior and posterior ends on each side, is a demi-facet (fig. 70, A, 4), which, together with that on the adjacent vertebra, forms an articulating surface for the capitulum of the rib. The neural arch is small and deeply notched behind for the passage of the spinal nerve. It is drawn out above into a very long neural spine (fig. 70, A, 1), whose base extends back over the succeeding vertebra and bears the downwardly-directed postzygapophyses (fig. 70, A, 6). The summit of the neural arch is deeply notched in front, and on each side of the notch are the prezygapophyses, which look almost vertically upwards. The transverse processes are short and blunt, and are flattened below (fig. 70, A, 3) for the articulation of the tubercula of the ribs.

The posterior three or four thoracic vertebrae differ much from the others. The centra are longer, the neural spines short and not directed backwards, the articular facets for the heads of the ribs are confined to the anterior end of the centrum of each vertebra, not overlapping on to the preceding vertebra. The transverse processes are small and irregular, and metapophyses and anapophyses are developed. The prezygapophyses also look more inwards, and the postzygapophyses more outwards than in the more typical thoracic vertebrae.

THE LUMBAR VERTEBRAE.

The =lumbar vertebrae= are seven in number, and their general characteristics have been already described. As a group they are characterised by their large size, and the great development of the transverse processes, metapophyses and neural spines.

THE SACRAL VERTEBRAE.

Three vertebrae are commonly found fused together, forming the =sacrum=; the divisions between the three being indicated by the foramina for the exit of the spinal nerves.

Of these three vertebrae, the first is much the largest, and is firmly united to the ilium on each side by a structure formed by the transverse processes and expanded ribs. In the adult this structure forms one continuous mass, but in the young animal a ventral portion formed by the rib is clearly distinguishable from a dorsal portion formed by the transverse process. All three have low neural spines. The anterior sacral vertebra bears a large pair of prezygapophyses, while the posterior one bears a small pair of postzygapophyses.

THE CAUDAL VERTEBRAE.

The =caudal vertebrae= are about nineteen in number. The earlier ones have well-developed neural arches, transverse processes, and zygapophyses, but as the vertebrae are followed back they gradually lose all their processes, and the neural arch as well, becoming at about the thirteenth from the end reduced to simple cylindrical centra.

B. THE SKULL.

The =skull= consists of the following three parts: (_a_) the cranium, with which are included the skeletal supports of the various special sense organs, and the bones of the face and upper jaw; (_b_) the lower jaw or mandible, which is movably articulated to the cranium, and (_c_) the hyoid.

(_a_) THE CRANIUM.

The cranium is a compact bony box, forming the anterior expanded portion of the axial skeleton. It has a longitudinal axis, the =craniofacial= axis around which the various parts are arranged, and this axis is a direct continuation of that of the vertebral column. Similarly the cavity of the cranium is a direct continuation of the spinal canal. The posterior part of the craniofacial axis, which has relations only with the cranium, is called the =basicranial axis=.

In the dog as in the other types previously described, the skull in its earliest stages is cartilaginous, containing no bone. In the adult, however, the cartilage is to a great extent replaced by bone, and in addition to this cartilage bone, membrane bone is largely developed, and intimately united with the cartilage bone to form one complete whole.

In the description of the dog's skeleton, as in those of the previous types, the names of the membrane bones are printed in italics, while those of the cartilage bones are printed in thick type.

Most of the numerous foramina perforating the skull walls will be described after the bones have been dealt with.

For purposes of description the cranium may be further subdivided into:--

1. The cranium proper or brain case.

2. The sense capsules.

3. The upper jaw.

1. THE CRANIUM PROPER OR BRAIN CASE.

Taking the membrane and cartilage bones together, they are seen to be more or less arranged in three segments, which however must not be regarded as homologous with the segments forming the vertebral column.

The =occipital segment= is the most posterior of the three, and consists of four cartilage bones, which in the adult are commonly completely fused together. They surround the great =foramen magnum= (fig. 75, 2) through which the brain and spinal cord communicate. Forming the lower margin of the foramen magnum is a large flat unpaired bone, the =basi-occipital= (fig. 75, 5). Above this on each side are the =exoccipitals=, whose sides are drawn out into a pair of downwardly-directed =paroccipital processes=, which are applied to the tympanic bullae[141]. The inner side of each exoccipital is converted into the large rounded =occipital condyle= (fig. 72, 13) by which the skull articulates with the atlas vertebra. The dorsal boundary of the foramen magnum is formed by a large unpaired flat bone, the =supra-occipital= (figs. 72 and 75, 1), which is continuous with a small bone, the _interparietal_, prolonged forwards between the parietal bones of the next segment.

In old animals the interparietal forms the hind part of a prominent ridge running along the mid-dorsal surface of the skull and called the =sagittal crest=, while the junction line of the occipital and parietal segments forms a prominent =occipital crest=.

The plane in which the bones of the occipital segment lie is called the occipital plane; the angle that it makes with the basicranial axis varies much in different mammals.

The =parietal segment= consists of both cartilage and membrane bones. It is formed of five bones, which are in contact with those of the occipital segment on the dorsal and ventral surfaces, while laterally they are separated by the interposition of the auditory bones, and to some extent of the squamosal. The =basisphenoid= (fig. 75, 6), an unpaired bone forming the ventral member of this segment, is the direct continuation of the basi-occipital. It tapers anteriorly, but is rather deep vertically, its upper or dorsal surface bearing a depression, the =sella turcica=, which lodges the pituitary body of the brain. From the sides of the basisphenoid arise the =alisphenoids= (fig. 75, 11) a pair of bones of irregular shape generally described as wing-like; each gives off from its lower surface a =pterygoid plate=, which is united in front with the palatine, and below with the pterygoid. The alisphenoids are united above with a pair of large nearly square bones, the _parietals_ (fig. 73, 2), which meet one another in the mid-dorsal line. The line of junction is frequently drawn out into a strong ridge, which forms the anterior part of the =sagittal crest=.

The =frontal segment=, which surrounds the anterior part of the brain, is closely connected along almost its whole posterior border with the parietal segment.

Its base is formed by the =presphenoid= (fig. 75, 12), a very deep unpaired bone, narrow and compressed ventrally, and with an irregular dorsal surface. The presphenoid is continuous with a second pair of wing-like bones, the =orbitosphenoids=. Each orbitosphenoid meets the alisphenoid behind, but the relations of the parts in this region are somewhat obscured by a number of large foramina piercing the bones, and also by an irregular vacuity, the =foramen lacerum anterius= or =sphenoidal fissure=, which lies between the orbitosphenoid and alisphenoid, separating the lateral parts of the parietal and frontal segments, in the same way as the space occupied by the auditory bones separates the lateral parts of the occipital and parietal segments. The orbitosphenoids pass obliquely forwards and upwards, and are united above with a second pair of large membrane bones, the _frontals_ (fig. 73, 3). The outer side of each frontal is drawn out into a rather prominent rounded =postorbital process= (fig. 73, 10), from which a ridge converges backwards to meet the sagittal crest. The anterior part of the frontal is produced to form the long nasal process, which is wedged in between the nasal and maxillae.

The cranial cavity is continuous in front with the =nasal= or =olfactory cavities=, but the passage is partially closed by a screen of bone, the =cribriform plate= (fig. 72, 5), which is placed obliquely across the anterior end of the cranial cavity, and is perforated by a number of holes through which the olfactory nerves pass. The plane of the cribriform plate is called the =ethmoidal plane=, and as was the case also with the occipital plane, the angle that it makes with the basicranial axis varies much in different mammals, and is of importance. The =olfactory fossa= in which lie the olfactory lobes of the brain, is partially separated from the =cerebral fossa=, or cavity occupied by the cerebral hemispheres, by ridges on the orbitosphenoids and frontals. The presphenoid is connected in front with a vertical plate formed partly of bone, partly of unossified cartilage; this plate, the =mesethmoid= (fig. 72, 7), separates the two olfactory cavities which lodge the olfactory organs. Its anterior end always remains unossified, and forms the septal cartilage of the nose.

The brain case may then, to use the words of Sir W.H. Flower, be described as a tube dilated in the middle and composed of three bony rings or segments, with an aperture at each end, and a fissure or space at the sides between each of them.

2. THE SENSE CAPSULES.

Each of the three special sense organs, of hearing, of sight, and of smell, is in the embryo provided with a cartilaginous or membranous protecting capsule; and two of these, the auditory and olfactory capsules, become afterwards more or less ossified, and intimately related to the cranium proper.

(1) =Bones in relation to the Auditory capsules.=

These bones lie on each side wedged into the vacuity between the lateral parts of the occipital and parietal segments; they are three in number, the =periotic=, the _tympanic_ and the _squamosal_.

The =periotic= is the most important of them, as it replaces the cartilaginous auditory capsule of the embryo, and encloses the essential organ of hearing. It commences to ossify from three centres corresponding to the pro-otic, epi-otic and opisthotic of lower skulls, such as those of the Turtle and Crocodile.

These ossifications however very early combine to form a single bone, the =periotic=, which nevertheless consists of two portions, the =petrous= and the =mastoid=, differing considerably from one another.

The =petrous portion= lies dorsally and anteriorly, and is much the more important of the two, as it encloses the essential part of the auditory organ. It forms an irregular mass of hard dense bone, projecting into the cranial cavity, and does not appear on the external surface at all. The =mastoid portion= lies ventrally and posteriorly, is smaller, and formed of less dense bone than is the petrous portion, from which it differs also in the fact that it appears on the surface of the skull, just external to the exoccipital. The petrous portion bears a ridge, which together with a ridge on the supra-occipital, and the =tentorium= (fig. 72, 21), a transverse fold of the dura mater[142], separates the large cerebral fossa from the =cerebellar fossa=, which is much smaller than the cerebral fossa and lies behind and partly beneath it. The plane of the tentorium is called the =tentorial plane=, and the angles that it makes with the basicranial axis and with the occipital and ethmoidal planes vary much in different mammals.

The periotic has its inner surface marked by important depressions, while both inner and outer surfaces are pierced by foramina. At about the middle of its inner surface are seen two deep pits, one lying immediately above the other. Of these the more ventral is a foramen, the =internal auditory meatus= (fig. 72, 20), through which the VIIth (facial) and VIIIth (auditory) nerves leave the cranial cavity, the facial nerve passing through the bone and afterwards leaving the skull by the =stylomastoid foramen= (fig. 75, VII), while the auditory passes to the inner ear. The more dorsal of the two pits is not a foramen but the =floccular fossa= (fig. 72, 23) which lodges the floccular lobe of the cerebellum. In some skulls another wide and shallow but fairly prominent depression is seen dorsal to and slightly behind the floccular fossa, this also lodges part of the cerebellum. Behind the internal auditory meatus, between the periotic and exoccipital is seen the internal opening of the =foramen lacerum posterius= (fig. 72, 22). The shape of this opening varies. The ventro-anterior border of the periotic is marked by a deep notch, the sides of which sometimes unite, converting it into a foramen.

On the outer side of the periotic, and clearly seen only after the removal of the tympanic, are two holes, the =fenestra ovalis= and the =fenestra rotunda=.

The _tympanic_ (figs. 72, 15 and 75, 4) is a greatly expanded boat-shaped bone, which forms the auditory bulla and lies immediately ventral to the periotic; it is separated from the periotic by the =tympanic cavity= into which the fenestra rotunda and the fenestra ovalis open.

There are several other openings into the tympanic cavity.

(_a_) On the external surface is a large oval opening, the =external auditory meatus= bounded by a thickened rim.

(_b_) Into the outer and anterior part of the cavity the outer end of the =Eustachian tube= opens; while the inner end passes through a foramen (fig. 75, 22) just external to the foramen lacerum medium, on its way to open into the pharynx.

(_c_) The internal carotid artery also enters the tympanic cavity by a canal which commences in the foramen lacerum posterius, and passes forwards to open on the inner side of the bulla. The artery then passes forwards, and barely appearing on the ventral surface of the cranium, enters the brain cavity through the foramen lacerum medium (fig. 75, 9).

Immediately behind the tympanic, between it and the mastoid process of the periotic and the paroccipital process of the exoccipital is the =stylomastoid foramen= (fig. 75, VIII).

Within the tympanic cavity are four small bones, the =auditory ossicles= (cp. fig. 74), called respectively the =malleus=, =incus=, =lenticular= and =stapes=; these together form a chain extending from the fenestra ovalis to the tympanic membrane.

The =malleus= has a somewhat rounded head (fig. 100, B, 1) which articulates with the incus, while the other end of the bone is drawn out into a long process, the =manubrium=, which lies in relation to the tympanic membrane. The head is also more or less connected by a thin plate of bone, the =lamella=, to another outgrowth, the =processus longus=. The =incus= (fig. 100, B, 3) is somewhat anvil-shaped, and is drawn out into a process which is connected with the =lenticular=, a nodule of bone interposed between the incus and the stapes, with which it early becomes united. The =stapes= (fig. 100, B, 2) is stirrup-shaped, consisting of a basal portion from which arise two =crura=, which meet and enclose a space, the =canal=.

The _squamosal_ (fig. 73, 7) is a large bone occupying much of the side wall of the cranial cavity, and articulating above with the parietal, and behind with the supra-occipital, while in front it overlaps the frontal and alisphenoid. But though it occupies so large a space on the outer wall, it forms very little of the internal wall of the skull, but is really like a bony plate attached to the outer surface of the cranial wall. The squamosal is drawn out into a strong forwardly-directed =zygomatic process= which meets the jugal or malar. The ventral side of the zygomatic process is hollowed out, forming the =glenoid fossa= (fig. 75, 8), a smooth laterally elongated surface with which the lower jaw articulates, while the hinder edge of the glenoid fossa is drawn out into a rounded =postglenoid process= (fig. 75, 23). The articulation is such as to allow but little lateral play of the lower jaw.

(2) =Bones in relation to the Optic capsules.=

The only bone developed in relation to the optic capsule on each side is the _lachrymal_. This is a small membrane bone lying between the frontal and palatine behind, and the maxillae and jugal in front. It is perforated by a prominent =lachrymal foramen= (fig. 73, 13) which opens within the orbit.

(3) =Bones in relation to the Olfactory capsules.=

In connection with the =olfactory capsules=, five pairs of bones are developed, two pairs being membrane bones, and three pairs cartilage bones.

Of membrane bones, the _nasals_ (fig. 73, 4) are a pair of long narrow bones, lying closely side by side, and forming the main part of the roof of the olfactory chamber. Their posterior ends overlap the frontals, and the outer margin of each is in contact with the nasal process of the frontal, and with the maxillae and premaxillae.

Lying immediately ventral to the nasals, and on each side of the perpendicular mesethmoid, are the =ethmoid= or =turbinal= bones, which have a curious character, being formed of a number of delicate plates intimately folded on one another. The posterior pair of these bones, the =ethmo-turbinals= (fig. 72, 10), are the larger, and form a mass of intricately folded lamellae attached behind to the cribriform plate, and passing laterally into two thin plates of bone, which abut on the maxillae. The uppermost lamella of each ethmo-turbinal is larger than the others and more distinct. It is sometimes distinguished as the =naso-turbinal=, and forms an imperfect lower boundary to a canal, which is bounded above by the nasals. In front of and somewhat below the ethmo-turbinals, lie another pair of bones of similar character, the =maxillo-turbinals= (fig. 72, 11).

The last bone to be mentioned in connection with the olfactory capsules is a membrane bone, the _vomer_ (fig. 72, 9). This is a slender vertically-placed bone, whose anterior part lies between the maxillo-turbinals, while behind it extends beyond the mesethmoid, so as to underlie the anterior part of the presphenoid. The anterior part of the vomer forms a kind of trough, while further back in the region of the ethmo-turbinals it sends out a pair of strong lateral plates, each of which, passing below the ethmo-turbinal, joins the side wall of the nasal cavity, and forms a partition dividing the nasal cavity into a lower =narial passage= and an upper =olfactory chamber=.

THE JAWS.

In the embryo both upper and lower jaws are formed of cartilaginous bars, but in the adult not only has the cartilage entirely disappeared, but even cartilage bone is absent, the jaws being formed of membrane bone.

3. THE UPPER JAW.

The bones of the upper jaw are closely connected with those of the cranium proper and olfactory capsules. The most posterior of them is the _pterygoid_ (fig. 75, 15), a thin vertically placed plate of bone, which articulates above with the basisphenoid, the presphenoid, and the strong pterygoid process of the alisphenoid. The ventral end of the pterygoid is drawn out into a small backwardly-projecting =hamular process=. In front the pterygoid articulates with the _palatine_, a much larger bone, consisting of (1) a vertical portion, which passes up to meet the orbitosphenoid and frontal, and sends inwards a plate which meets the presphenoid and vomer, forming much of the roof of the posterior part of the narial passage; and (2) a strong horizontal portion, the =palatal process= (fig. 75, 16), which passes inwards and meets its fellow in the middle line, forming the posterior part of the bridge of bone supporting the hard palate. The palatal process is continuous in front, with a large bone, the _maxillae_, which, like the palatine, consists of vertical and horizontal portions. The vertical, or =facial portion= (fig. 73, 5), is the largest, and constitutes the main part of the side of the face in front of the orbit, forming also the chief part of the outer wall of the nasal cavity. It is continuous in front with the premaxillae, above with the nasal and frontal, and behind with the lachrymal, jugal, and palatine. The horizontal, or =palatal portion= (fig. 75, 17), forms the anterior part of the bony plate supporting the hard palate, and meets its fellow in a long straight symphysis. The junction line between the palatal and facial portions is called the =alveolar border=, and along it are attached the canine, premolar, and molar teeth.

The anterior part of the upper jaw on each side is formed by a small bone, the _premaxillae_, which bears the incisor teeth. It, like the maxillae, has a palatal portion (fig. 75, 20), which meets its fellow in the middle line, and an ascending portion, which passes backwards as the =nasal process=, tapering regularly and lying between the nasal and the maxillae. The two premaxillae form the outer and lower borders of the anterior nares. The last bone to be mentioned in connection with the upper jaw and face is the _jugal_ or _malar_ (figs. 73, 8, and 75, 14), a strong bone which forms the anterior half of the zygomatic arch. It is firmly united in front to the maxillae, and behind meets the zygomatic process of the squamosal, being drawn out dorsally into a short =postorbital process= at the point of meeting. This process lies immediately below the postorbital process of the frontal, and if the two met, as they do in some mammals, they would partially shut off the orbit from a larger posterior cavity, the =temporal fossa=. In the living animal a ligament unites the two postorbital processes.

(_b_) THE LOWER JAW OR MANDIBLE.

This consists of two elongated symmetrical halves, the =rami=, which are united to one another at the median symphysis in front, while behind they diverge considerably, and each articulates with the glenoid surface of the corresponding squamosal. In young animals the rami are united at the symphysis by fibrous tissue, but in old animals they sometimes become fused together. The upper or alveolar border bears the teeth, and behind them is drawn out into a high laterally compressed =coronoid process= (fig. 72, 24), which is hollowed on its outer surface. Immediately behind the coronoid process is the transversely elongated =condyle= (fig. 72, 25), which fits into the glenoid cavity in such a way as to allow free up and down movement of the jaw, with but little rolling motion. The posterior end of the jaw below the condyle forms a short rounded process, the =angle= (fig. 72, 26). Two prominent foramina are to be seen in the lower jaw. These are firstly the =inferior dental foramen= (fig. 72, 28), which lies on the inner surface below the coronoid process; through it an artery and a branch of the fifth nerve enter to supply the teeth, and secondly the =mental foramen=, which lies on the outer side near the anterior end, and through which a branch of the same nerve emerges.

(_c_) THE HYOID.

The =Hyoid= of the dog consists of a transverse median piece, the =basi-hyal=[1] (fig. 72, 32), from which arise two pairs of =cornua=. The =anterior cornu= is much the longer of the two, and consists principally of three short separate ossifications, placed end to end and called respectively the =cerato-hyal=[143], =epi-hyal=, and =stylo-hyal=. All of them are short rods of bone, contracted in the middle, and expanded at the ends, where they are tipped with cartilage. The cerato-hyal (fig. 72, 31) lies next to the basi hyal. The stylo-hyal is terminated by a much smaller bone, the =tympano-hyal=, which lies in a canal between the tympanic and periotic, and is ankylosed to the periotic just to the anterior and inner side of the stylomastoid foramen.

The =posterior cornu= of the hyoid is much smaller than the anterior; it consists of a short bone, the =thyro-hyal= (fig. 72, 33), which connects the basi-hyal with the thyroid cartilage of the larynx.

FORAMINA OF THE SKULL.

The foramina, or apertures perforating the walls of the skull, are very numerous, and may either be due to holes actually penetrating the bone, or may be small vacuities between the margins of two elsewhere contiguous bones.

They may be divided into two groups, the first including

I. The holes through which the =twelve cranial nerves= leave the cranial cavity.

_a._ The most anterior of these nerves, the olfactory, leaves the skull by a number of small holes piercing the =cribriform plate= (fig. 72, 5).

_b._ The second, or optic, passes out by a large hole, the =optic foramen= (fig. 75, II) piercing the orbitosphenoid. The optic foramen is the most anterior of the three prominent holes seen within and immediately behind the orbit.

_c._ The third, fourth, and sixth nerves, i.e. those supplying the eye muscles, and with them the first or ophthalmic branch of the large fifth or trigeminal nerve, pass out by a large hole, the =foramen lacerum anterius= (fig. 75, III, IV, V{1}, VI), which, as has been already mentioned, lies between the orbitosphenoid and alisphenoid.

_d._ Immediately behind the foramen lacerum anterius, the alisphenoid is perforated by a prominent round hole, the =foramen rotundum= (fig. 75, V{2}), through which the second branch of the trigeminal nerve passes out.

_e._ A quarter of an inch further back there is another prominent hole, the =foramen ovale= (fig. 75, V{3}), through which the third branch of the trigeminal nerve leaves the cranium.

_f._ The seventh or facial nerve, as already mentioned, leaves the cranial cavity and enters the auditory capsule, through an opening in the periotic called the =internal auditory meatus=, while it finally leaves the skull by the =stylomastoid foramen= (fig. 75, VII), which lies between the tympanic bulla, the paroccipital process, and the mastoid portion of the periotic.

_g._ The eighth or auditory nerve on leaving the cranial cavity, passes with the facial straight into the auditory capsule through the =internal auditory meatus= (fig. 72, 20). It is then distributed to the organ of hearing.

_h._ The ninth, tenth and eleventh nerves leave the skull through the =foramen lacerum posterius= (fig. 75, IX, X, XI), a large space lying between the auditory bones and the exoccipital.

_i._ Finally, the twelfth nerve, the hypoglossal, passes out through the prominent =condylar foramen= (fig. 75, XII), which perforates the exoccipital just behind the foramen lacerum posterius.

II. OTHER OPENINGS IN THE SKULL.

_a._ The =anterior narial opening= lies at the anterior end of the skull, and is bounded by the premaxillae and nasals. In the natural condition it is divided into two by a vertical partition, formed by the =narial septum=, the anterior unossified part of the mesethmoid.

_b._ Penetrating the middle of the maxillae at the side of the face is the rather large =infra-orbital foramen= (fig. 73, 11), through which part of the second branch of the trigeminal nerve passes out from the orbit to the side of the face.

_c._ Several foramina are seen perforating the anterior part of the orbit. The most dorsal of these, perforating the lachrymal bone, is the =lachrymal foramen= (fig. 73, 13). Lying below and slightly external to this is a large foramen, through which part of the second branch of the trigeminal enters on its way to the infra-orbital foramen and so to the side of the face. Lastly, lying below these, and perforating the palatine, are two closely apposed foramina, the =internal orbital foramina=, through which part of the first or ophthalmic branch of the trigeminal nerve leaves the orbit, passing into the nasal cavity.

_d._ The anterior part of the palate between the premaxillae and the maxillae is perforated by a pair of long closely apposed apertures, the =anterior palatine foramina= (fig. 75, 19). They transmit part of the trigeminal nerve.

_e._ Towards the posterior part of the palate are two pairs of small =posterior palatine foramina= (fig. 75, 18). These perforate the palatine and transmit branches of the trigeminal nerve and certain blood-vessels.

_f._ The =posterior narial opening= is bounded chiefly by the palatines.

_g._ The =alisphenoid canal= (fig. 75, 21) is a short canal penetrating the base of the alisphenoid bone, and transmitting the external carotid artery. It lies between the foramen rotundum and the foramen ovale.

_h._ Between the auditory bulla and the foramen ovale are seen two openings. The more external of these is the opening of the =Eustachian canal= (fig. 75, 22), which communicates with the tympanic cavity. The more internal is the =foramen lacerum medium= (fig. 75, 9), through which the internal carotid enters the cranial cavity.

_i._ The =external auditory aperture= (fig. 75, 7) is a large opening with rough edges at the outer side of the tympanic bulla.

_j._ Between it and the glenoid surface of the squamosal is the =postglenoid= foramen (fig. 75, 10) through which a vein passes out.

_k._ Lastly, there is the great =foramen magnum= (fig. 75, 2), between the occipital condyles. Through it the brain and spinal cord communicate.

C. THE RIBS AND STERNUM.

These, together with the thoracic vertebrae, form the skeletal framework of the thorax. Each rib is a curved rod, which at its dorsal end is movably articulated to the vertebra, and at its ventral end is either connected with the sternum, or ends freely. In the dog there are thirteen pairs of ribs, nine pairs of which are directly connected with the sternum, while the remaining four end freely and are known as =floating ribs=. Each rib is obviously divided into two parts, a dorsal or =vertebral part=, and a ventral or =sternal part=. The vertebral portion, which forms about two-thirds of the whole rib, is a flattened, regularly curved rod, completely ossified. Its dorsal end is rounded, forming the =head= or =capitulum=, which articulates with a concave surface furnished partly by the corresponding vertebra and partly by the vertebra next in front. The last three or four however articulate with one vertebra only. A short way behind the capitulum on the dorsal side of the rib is a rounded outgrowth, the =tubercle= or =tuberculum=, by means of which the rib articulates with the transverse process. The portion of the rib between the head and the tubercle is known as the =neck=. The =sternal portion= of the rib (fig. 76) is a short bar of calcified or imperfectly ossified cartilage, about one-third of the length of the corresponding bony portion. The anterior sternal ribs are somewhat more cartilaginous than the posterior ones. The vertebral portions increase in length from the first which is very stout, and has the capitulum and tuberculum very distinct, to about the eighth or ninth; afterwards they gradually diminish in size. The first nine to eleven have the capitula and tubercula separate, afterwards they gradually merge together.

THE STERNUM.

This is an elongated cylindrical structure lying in the mid-ventral wall of the thorax, and is divided into eight segments or =sternebrae=. The anterior segment, the =presternum= (fig. 76, 1) or =manubrium sterni= is expanded in front; the next six segments, which, together form the =mesosternum= are elongated, somewhat contracted in the middle and expanded at the ends. The last segment or =xiphisternum= (fig. 76, 4) is long and narrow, and terminates in a flattened expanded plate of cartilage. The first pair of sternal ribs articulate with the sides of the presternum, and the remaining pairs between the successive sternebrae. Between the last sternebra and the xiphisternum two pairs articulate. Development shows that the sternum is formed by the union in the middle line of two lateral portions; this can be well seen in the presternum and xiphisternum of the puppy, but no traces of this median division remain in the adult dog.

2. THE APPENDICULAR SKELETON.

The appendicular skeleton consists of the bones of the anterior and posterior limbs, and of their respective supports, the pectoral and pelvic girdles.

THE PECTORAL GIRDLE.

The =pectoral girdle= lies external to the ribs, and has no bony attachment to the axial skeleton. In almost all Mammalia it is, as compared with that in Sauropsids, very incomplete; and in the dog it is even more reduced than in the majority of Mammalia. The dorsal portion or =scapula= is well developed, but the ventral portion is almost entirely absent.

The =scapula= is somewhat triangular in shape, the apex being directed downwards and forwards, and being expanded to form the shallow =glenoid cavity= with which the head of the humerus articulates. The inner surface of the scapula is nearly flat, while the outer is drawn out into a very prominent ridge, the =spine=, which, arising gradually near the dorsal end, runs downwards, dividing the surface into two nearly equal parts, the =prescapular= and =postscapular fossae=, and ends in a short blunt process, the =acromion=. The anterior border of the scapula is somewhat curved, and is called the =coracoid border=; it is terminated ventrally by a slight blunt swelling, the =coracoid process=, which ossifies from a different centre from the rest of the scapula, and is probably the sole representative of the =coracoid=. The dorsal or =suprascapular border= of the scapula is rounded, while the posterior or =glenoid border= is nearly straight. The clavicle[144] or collar bone, which in a large proportion of mammals is well seen, in the dog is very imperfectly developed; it is short and broad, and is suspended in the muscles, not reaching either the scapula or sternum.

THE ANTERIOR LIMB.

The anterior limb of the dog is divisible into the usual three portions, the =brachium= or =upper arm=, the =antibrachium= or =fore-arm=, and the =manus= or =wrist= and =hand=.

The =brachium= or =upper arm= includes only a single bone, the =humerus=.

The =humerus= is a stout elongated bone, articulating by its large proximal =head= (fig. 77, 1) with the glenoid cavity of the scapula, and at its distal end by the =trochlea= with the bones of the fore-arm. The head passes on its inner side into an area roughened for the attachment of muscles and called the =lesser tuberosity= (fig. 77, 2); while in front it is divided by the shallow =bicipital groove= from a large roughened area, the =greater tuberosity= (fig. 77, 3), which is continued as a slight roughened ridge, extending about one-third of the way down the outer side of the shaft. This ridge, which in many animals is much more strongly developed than it is in the dog, is called the =deltoid ridge=. The =trochlea= (fig. 77, 5) at the distal end of the bone is a pulley-like surface, elevated at the sides and grooved in the middle. It articulates with the radius and ulna of the fore-arm. On each side of it are slight roughened projections, the =internal= and =external condyles= (fig. 77, 7). In the cat and many other animals there's a foramen, the =ent-epicondylar foramen= above the internal condyle, but in the dog this is not developed. Passing up the shaft from the external condyle is a slight ridge, the =supinator= or =ectocondylar ridge=; this is better developed in many mammals. Immediately above the trochlea in front and behind are the deep =supra-trochlear fossae=, which communicate with one another through the =supra-trochlear foramen= (fig. 77, 8). The posterior of these, the =olecranon fossa=, is much the deeper, and receives the olecranon process of the ulna when the arm is extended. The head and tuberosities of the humerus ossify from one centre, the shaft from a second, and the trochlea and condyles from a third.

The =fore-arm= or =antibrachium= contains two bones, the =radius= and =ulna=; they are immovably articulated with one another, but not fused. The pre-axial bone, the =radius= (fig. 77, B), which lies more or less in front of the ulna, is external to the ulna at its proximal end, and at its distal end is internal to that bone. It articulates with the external portion of the trochlea, while the ulna articulates with the internal portion. It is a straight bone with its distal end slightly larger than its proximal end. The proximal end articulates with the trochlea, the distal end with the bones of the carpus.

The postaxial bone, the =ulna= (fig. 77, C), has the proximal end much enlarged, forming the =olecranon= (fig. 77, 11), and tapers gradually to the distal end. Near its proximal end the ulna is marked by a deep =sigmoid notch=, which bears on its inner side a concave surface (fig. 77, 12) for articulation with the trochlea. The pointed proximal end of the sigmoid notch is called the =coronoid process=. Somewhat in front of and below the sigmoid notch is a smaller hollow (fig. 77, 13), with which the radius articulates.

In the young animal the ends of both radius and ulna are seen to ossify from centres different from those forming the shafts. The epiphyses forming both ends of the radius, and the distal end of the ulna are large, while that at the proximal end of the ulna is small, and forms only the end of the olecranon.

The =Manus= is divided into

_a._ The =carpus= or =wrist=, formed of a group of small bones.

_b._ The =hand=, which includes firstly some elongated bones, the =metacarpals=, forming what corresponds to the palm of the hand, and secondly the phalanges, which form the =fingers=.

The =Carpus= or =wrist=. The carpus of the dog consists of seven small bones, arranged in a proximal row of three, and a distal row of four. It differs much from the simpler type met with in the newt. The largest bone of the proximal row is the =scapho-lunar= (fig. 80, 1), formed by the fused =scaphoid= (radiale), =lunar= (intermedium), and =centrale=; it has a large convex proximal surface for articulation with the radius, and articulates distally with the trapezium, trapezoid, and magnum, and internally with the cuneiform. The =cuneiform= (ulnare) (fig. 80, 2) has a posterior rounded surface articulating with the ulna; it articulates in front with the unciform, and internally with the =pisiform= (fig. 80, 7), which is a comparatively large sesamoid bone on the ulnar side of the carpus. Frequently also there is a small sesamoid bone on the radial side of the carpus. The =trapezium= (carpale 1), =trapezoid= (carpale 2), and =magnum= (carpale 3) (fig. 80, 5) are all small bones, and support respectively the first, second, and third metacarpals. The =unciform= (carpalia 4 and 5) (fig. 80, 6) is larger, and supports the fourth and fifth metacarpals.

The hand has five =digits=, each consisting of an elongated =metacarpal=, followed by =phalanges=, the last of which, the =ungual phalanx=, is pointed and curved, and bears the claw. Each of the metacarpals is seen in the young animal to have its distal end formed by a prominent epiphysis, and each of the phalanges, except those bearing the claws, has a similar epiphysis at its proximal end.

The =pollex= (fig. 80, A, I ) is far shorter than the other digits, and normally does not touch the ground in walking. It has only two phalanges, while each of the other digits has three. A pair of small sesamoid bones are developed on the ventral or flexor side of the metacarpo-phalangeal articulations of all the digits except the pollex. Frequently similar sesamoid bones occur also on the dorsal side of the phalangeal articulations.

THE PELVIC GIRDLE.

The =pelvic girdle= consists of two halves, which lie nearly parallel to the vertebral column.

Each half is firmly united to its fellow in a ventral symphysis behind, and is in front expanded and united to the sacrum. Each half or =innominate bone= is seen in the young animal to consist of four distinct parts, the =ilium= or dorsal element, the =pubis= or anterior ventral element, the =ischium= or posterior ventral element, and a small fourth part, the =acetabular= or =cotyloid= bone, wedged in between the three others. These parts, though all distinct in the young animal, are in the adult so completely fused that their respective boundaries cannot be distinguished. At about the middle of the outer surface of the innominate bone is a very deep cavity, the =acetabulum= (fig. 78, A, 1) with which the head of the femur articulates; all the bones except the pubis take part in its formation.

The =ilium= is a rather long bone, expanded in front and contracted behind; it forms about half the acetabulum. On its inner or =sacral surface= (fig. 78, 4) is a large roughened patch for articulation with the sacrum; its outer or =gluteal surface= is concave. The posterior part of the bone is flattened below, forming the narrow =iliac surface= (fig. 78, A, 5).

The =ischium= (fig. 78, 9) is a wide flattened bone forming the posterior part of the innominate bone. It meets the pubis ventrally, but is separated from it for the greater part of its length by the large =obturator= or =thyroid foramen= (fig. 78, 2). At its posterior end externally it bears a rather prominent roughened =ischial tuberosity= (fig. 78, A, 10). The ischium meets its fellow in a ventral symphysis, and forms about one-third of the acetabulum.

The =pubis= (fig. 78, 12) is smaller than either the ischium or ilium; it does not take part in the formation of the acetabulum, and like the ischium, meets its fellow in a ventral symphysis. The =acetabular bone= (fig, 78, B, 14) is small and triangular, and is wedged in between the other three. It forms about one-sixth of the acetabulum.

THE POSTERIOR LIMB.

The =posterior limb=, like the anterior, is divisible into three parts; these are the =thigh=, the =crus= or =shin=, and the =pes=.

The =thigh= contains only a single bone, the =femur=.

The =femur= is a long straight bone with a nearly smooth shaft and expanded ends. The proximal end bears on its inner side the large rounded =head= (fig. 79, A, 1) which articulates with the acetabulum. External to the head and divided from it by a deep pit is a large rough outgrowth, the =great trochanter= (fig. 79, 3). The deep pit is the =trochanteric= or =digital fossa=. On the inner side below the head is a smaller roughened surface, the =lesser trochanter=. The lower or distal end of the bone bears two prominent rounded surfaces, the =condyles=, which articulate with the tibia. They are separated from one another by the deep =intercondylar notch=, which is continued above and in front as a shallow groove, lodging a large sesamoid bone, the =patella= or =knee-cap=. At the back of the knee-joint are a pair of smaller sesamoids, the =fabellae= (fig. 79, 7).

In the young animal there are three epiphyses to the shaft of the femur, one forming the head, one the great trochanter, and one the distal end.

The =crus= or =shin= contains two bones, the =tibia= and =fibula=. The =tibia= is a fairly thick straight bone, expanded at both ends, especially at the head or proximal end. The proximal end is triangular in cross section, and bears two facets for articulation with the condyles of the femur. The anterior surface of the proximal end of the tibia is marked by the strong =cnemial crest= (fig. 79, 8), which runs some way down the shaft. The distal end of the tibia articulates with the astragalus by an irregular, somewhat square surface.

The shaft of the tibia ossifies from one centre, the distal end from a second, and the proximal end from two more.

The =fibula= (fig. 79, C) is a distinct but very slender bone, somewhat expanded at both ends. It lies external to the tibia and articulates by its proximal end with the head of the tibia, and by its distal end with the calcaneum. Its shaft and proximal end ossify from one centre, and its distal end from a second.

The =Pes=.

The structure of the =pes= corresponds closely with that of the manus. It is divided into:--

_a._ The =tarsus= or =ankle= formed of a group of small bones.

_b._ The =foot=, which includes, firstly, some elongated bones, the =metatarsals=, forming what corresponds to the sole of the foot, and secondly the =phalanges=, which form the toes.

The =Tarsus=. The tarsus of the dog consists of seven bones arranged in two rows, of two and four respectively, with a =centrale= between them. The two bones of the proximal row are the =astragalus= and =calcaneum=.

The =astragalus= (fig. 80, 10) corresponds to the fused =tibiale= and =intermedium= of the typical tarsus. Its proximal end is much wider than its distal end, and forms a large rounded =condyle= articulating with the tibia, while its posterior end meets the navicular. It lies to the dorsal side of the foot.

The =calcaneum= (fibulare) (fig. 80, 11), the thickest bone in the pes, lies somewhat behind, and to the outer side of the astragalus. It articulates with the astragalus and fibula, and is drawn out behind into a long rounded process, which forms the heel, and is in the young animal terminated by an epiphysis. Between the proximal and distal rows of tarsals is the =navicular= (centrale) (fig. 80, 12), a somewhat flattened and square bone articulating with the astragalus.

The distal row of tarsals consists of four bones. The =internal cuneiform= (tarsale 1) is a smooth flattened bone lying to the inner side of the foot; it articulates with the first metatarsal and with the navicular. The =middle cuneiform= (tarsale 2) (fig. 80, 13) is a still smaller bone, lying external to the internal cuneiform. It articulates with the second metatarsal and with the navicular. The =external cuneiform= (tarsale 3) (fig. 80, 14) is a larger, somewhat square bone lying external to the middle cuneiform. It articulates with the third metatarsal and with the navicular. The =cuboid= (tarsalia 4 and 5) (fig. 80, 15) is a considerably larger bone lying to the outer side of the foot. It articulates with the fourth and fifth metatarsals and with the calcaneum.

The pes has sometimes five digits, sometimes four, the hallux being absent. Even when present the =hallux= (fig. 80, =B, I=) is commonly much reduced, and may be quite vestigial, and represented only by a small nodular metatarsal.

Each of the other digits consists of a long metatarsal, which in the young animal has a prominent epiphysis at its distal end, and of three phalanges. The proximal and middle phalanges have epiphyses at their proximal ends, while the distal phalanx is without epiphyses and is claw-shaped.

FOOTNOTES:

[139] W. Ellenberger and H. Baum, _Anatomie des Hundes_, Berlin, 1891.

[140] T.H. Huxley, "Dental and cranial characters of the Canidae," _P.Z.S._ 1880.

[141] See p. 392.

[142] The dura mater is a membrane which lines the cranial cavity and is formed of tough connective tissue.

[143] These are not strictly homologous with the basi-hyal and cerato-hyal of the Dogfish.

[144] See note to p. 25.