CHAPTER XI.

THE SKELETON OF THE FROG[55] (_Rana temporaria_).

I. EXOSKELETON.

The skin of the frog is smooth and quite devoid of scales or other exoskeletal structures. The only exoskeletal structures met with in the frog are:--

1. The =teeth=, which are most conveniently described with the endoskeleton.

2. The horny covering of the calcar or prehallux (see p. 167).

II. ENDOSKELETON.

The endoskeleton of the adult frog consists partly of cartilage, partly of bone and each of these types of tissue occurs in two forms. The cartilage may be hyaline, as in the omosternum and xiphisternum, or may be more or less calcified as in part of the suprascapula and the epiphyses of the limb bones. The bone may be cartilage bone, or membrane bone.

The skeleton is divisible into an =axial portion= consisting of the skull, vertebral column, and sternum, and an =appendicular portion= consisting of the skeleton of the limbs and their girdles.

1. THE AXIAL SKELETON.

A. THE VERTEBRAL COLUMN.

The vertebral column is a tube, formed of a series of ten bones which surround and protect the spinal cord. Of these ten bones nine are vertebrae, while the tenth is a straight rod, the =urostyle=, and is almost as long as all the vertebrae put together. The second to eighth vertebrae inclusive have a very similar structure, but the first and ninth differ from the others.

Any one of the second to eighth vertebrae forms a bony ring with a somewhat thickened floor, the =centrum= or body, which articulates with the centra of the immediately preceding and succeeding vertebrae. The articulating surfaces are covered with cartilage and are procoelous, or convex in front and concave behind. The eighth vertebra is however amphicoelous or biconcave. The centrum of each vertebra encloses an isolated vestige of the notochord. The =neural arch= forms the roof and sides of the neural canal, which is very spacious in the anterior vertebrae, but becomes more depressed in the posterior ones. The arch bears the =neural spine=, a low median ridge of variable character, and is drawn out in front and behind, forming the two pairs of articulating surfaces or =zygapophyses= by means of which the vertebrae are attached together. Of these the anterior articulating surfaces or =prezygapophyses= look upwards and slightly inwards, while the posterior articulating surfaces or =postzygapophyses= look downwards and slightly outwards. The sides of the neural arches are drawn out into a pair of prominent =transverse processes=. Those of the second vertebra look somewhat forwards, those of the third look directly outwards or somewhat forwards, while those of the fourth, fifth, and sixth are directed slightly backwards, and those of the seventh and eighth nearly straight outwards. All the transverse processes are terminated by very small cartilaginous =ribs=.

SPECIAL VERTEBRAE.

The =first vertebra= is a ring-like structure with a much depressed centrum. It bears in front two oval concave surfaces for articulation with the condyles of the skull, while the centrum is terminated behind by a prominent convex surface. There are as a rule no transverse processes, and the postzygapophyses look downwards and outwards. Occasionally however transverse processes do occur. Projecting forwards from the centrum is a minute process better developed in the Newt. This resembles an odontoid process, and it has hence been supposed that the first vertebra is homologous with the axis of mammalia, and that the atlas of the frog is fused with the skull.

The =ninth vertebra= has very stout transverse processes directed backwards and somewhat upwards. They articulate with the pelvic girdle and hence this vertebra is regarded as the =sacrum=. The neural arch is much depressed, the centrum is convex in front and bears on its posterior surface two short rounded processes for articulation with the urostyle.

The =urostyle= is a long rod-like bone forming the posterior unsegmented continuation of the vertebral column. It is probably equivalent to three vertebrae, the tenth, eleventh, and twelfth fused together, and to an unsegmented rod of cartilage which lies ventral to the notochord. The anterior end is expanded and bears two concave articular surfaces by means of which it articulates with the sacrum. A prominent ridge runs along the dorsal surface, but gradually diminishes when traced back. The anterior portion contains a canal which is a continuation of the neural canal. At a point not far from the anterior end, this canal communicates with the exterior by a pair of minute holes which correspond with the intervertebral foramina.

B. THE SKULL[56].

The skull of the Frog consists of three principal parts:--

(1) an axial part, the =cranium proper=, which encloses the brain. To it are firmly fused

(2) the =capsules of the olfactory and auditory sense organs=,

(3) lastly there is the =hyoid apparatus= and the =skeleton of the jaws=.

The skull is by no means so completely ossified as is the vertebral column, but in addition to the cartilage bone, there is a great development of membrane bone in connection with it.

The skull has a peculiarly flattened and expanded form depending on the wide lateral separation of the jaws from the cranium.

(1) THE CRANIUM PROPER or Brain case.

This is an unsegmented tube, which is widest behind. It remains to a considerable extent cartilaginous, but is partly converted into cartilage bone, partly sheathed in membrane bone. Its roof is imperfect, being pierced by three holes or =fontanelles=, one large anterior fontanelle (fig. 25, A, 9), and two smaller posterior fontanelles (fig. 25, A, 10).

The cartilage bones of the cranium proper are the two =exoccipitals= and the =sphenethmoid=.

The =exoccipitals= (figs. 24, 25, and 26, 6) are a pair of irregular bones bounding the foramen magnum at the posterior end of the skull. They almost completely surround the foramen magnum, and bear a pair of oval convex surfaces, the =occipital condyles=, with which the first vertebra articulates. The bones generally called the exoccipitals of the frog include the =epi-otic= and =opisthotic= elements of many skulls, in addition to the exoccipitals.

The patch of unossified cartilage immediately external to the occipital condyle is pierced by two small foramina, through which the ninth and tenth nerves leave the cranial cavity. The ninth nerve passes through the more external of these foramina, the tenth through the one nearer the condyle. The foramina lie however very close together and are sometimes confluent. The cranial walls for a considerable distance in front of the occipitals are unossified, but the anterior end of the cranial cavity is encircled by another cartilage bone, the =sphenethmoid= (figs. 24 and 25, 1) or girdle bone. This partly corresponds to the orbitosphenoids of the Newt's skull. Anteriorly it is pierced by a pair of small foramina through which the ophthalmic branches of the trigeminal nerve pass out.

The anterior part of the cranial cavity is divided into two halves by a vertical plate, the =mesethmoid=. Some little distance behind the sphenethmoid the ventro-lateral walls of the cartilaginous cranium are pierced by a pair of rather prominent holes, the =optic foramina= (figs. 24 and 25, B, II), and at a similar distance further back, occupying a kind of notch in the pro-otic are the large =trigeminal foramina=, through which the fifth and seventh nerves leave the cranium. Between the trigeminal and optic foramina are the very small foramina for the sixth nerves (fig. 25, B, VI).

The _membrane bones_ of the cranium proper include the _fronto-parietals_ and the _parasphenoid_.

The _fronto-parietals_ (figs. 24 and 26, A, 2) form a pair of long flat bones closely applied to one another in the middle line, the line of junction being the =sagittal suture=. They cover over the fontanelles and overlap the sphenethmoid in front.

The _parasphenoid_ (figs. 24 and 26, B, 7) is a bone shaped like a dagger with a very short handle. It lies on the ventral surface of the cranium, the blade being directed forwards and underlying the sphenethmoid; its lateral processes underlie the auditory capsules.

(2) THE SENSE CAPSULES.

The sense capsules are cartilaginous or bony structures which surround the olfactory and auditory organs and are closely united to the cranium.

The =auditory capsules= are fused with the sides of the posterior end of the cranium just in front of the exoccipitals. They are largely cartilaginous, but include in their anterior walls a pair of irregular cartilage bones, the =pro-otics= (figs. 24 and 25, 8). The cartilaginous area lying ventral to the pro-otic and external to the exoccipital is pierced by a rather prominent hole, the =fenestra ovalis=, which forms a communication between the internal ear cavity, and a space the tympanic cavity, which lies at the side of the head, and is bounded externally by the tympanic membrane. The fenestra ovalis is occupied by a minute cartilaginous structure, the =stapes=, and articulated partly to this and partly to a slight recess in the pro-otic is the =columella= (fig. 25, B, 5), a rod in part bony and in part cartilaginous, whose outer end is attached to the tympanic membrane. The columella and stapes are together homologous with the mammalian auditory ossicles and with the hyomandibular of Elasmobranchs. Sometimes the term columella is used to include the whole ossicular chain,--the columella together with the stapes.

The =olfactory= or =nasal capsules= (fig. 25, B, 12) are fused with the anterior end of the cranium and differ from the auditory capsules in being to a great extent unossified. There are however two pairs of membrane bones developed in connection with them, the _vomers_ and the _nasals_. They are drawn out into three pairs of cartilaginous processes, on the dorsal surface into the =prenasal= and =alinasal= processes which bound the external nares, and on the ventral surface towards the middle line into the forwardly-projecting =rhinal= processes.

The _nasals_ (figs. 24 and 26, 11) form a pair of triangular bones lying dorsolaterally in front of the fronto-parietals. Their bases are turned towards one another and their apices are directed outwards and backwards. They correspond in position with the prefrontals of the reptilian skull as well as with the nasals.

The _vomers_ are a pair of irregular bones lying on the ventral surface of the olfactory capsules. Each bears on its inner and posterior angle a group of minute pointed teeth, while its outer border is drawn out into three or four small slightly diverging processes, the two posterior of which form the inner boundary of the =posterior nares= (fig. 24, B, 15).

(3) THE JAWS.

The =upper jaw= consists of a rod of cartilage connected with the cranium near its two ends, but widely separated from it in the middle. It is almost completely overlain by membrane bone. With its posterior end the lower jaw articulates.

The membrane bones of the upper jaw include first the _premaxillae_, a small bone meeting its fellow in the middle line, and forming the extreme anterior end of the upper jaw. It gives off on its dorsal surface a backwardly-projecting process. It is connected behind with the _maxillae_ (figs. 24 and 26, 10), a long flattened bone which forms the greater part of the margin of the upper jaw, and gives off near its anterior end a short process which projects upwards and meets the nasal.

Both maxillae and premaxillae are grooved ventrally, and bear, attached to the outer more prominent margin of the groove, a row of minute conical teeth. These teeth are =pleurodont=, that is, are ankylosed by their bases and outer sides to the margin of the jaw. Each tooth is a hollow cone, the basal part of which is formed of bone, the apical part of dentine, capped by a very weak development of enamel.

The posterior end of the maxillae is overlapped by a small bone, the _quadratojugal_ (figs. 24 and 26, 9), whose posterior end forms part of the articular surface for the lower jaw. Just behind the quadratojugal there is a small unossified area which lies at the angle of the mouth, and is connected by a narrow bar of cartilage with the cranium; this forms the =quadrate= (figs. 24 and 26, 19). A backwardly-directed outgrowth from the cartilaginous bar more or less completely surrounds the tympanic membrane, forming the tympanic ring. When followed back the maxillae and quadratojugal diverge further and further from the cranium, till the angle of the mouth comes to be separated from the foramen magnum by a space nearly double the width of the cranium. This space is bridged over to a considerable extent by two triradiate bones, the _pterygoid_ and _squamosal_.

The _pterygoid_ (figs. 24 and 26, 3) is a large bone, whose anterior limb runs forwards meeting the maxillae and palatine; while its inner limb meets the auditory capsule and parasphenoid, and its outer limb runs backwards and outwards to the angle of the mouth. The _palatine_ is a small transversely-placed bone, which connects the pterygoid with the anterior part of the sphenethmoid. The _squamosal_ (figs. 24 and 26, 4) is a T-shaped bone whose anterior arm is pointed and passes forwards to meet the pterygoid. The posterior upper arm is closely applied to the pro-otic, while the posterior lower arm meets the pterygoid and quadratojugal at the angle of the jaw, and surrounds the narrow cartilaginous bar of the quadrate which goes to join the cranium. The squamosal is probably homologous with the squamosal together with the pre-opercular of Bony Ganoids.

The quadrate and squamosal form the =suspensorium= by which the lower jaw is connected with the cranium.

The =lower jaw= or =mandible= consists of a pair of cartilaginous rods (=Meckel's cartilages=) in connection with each of which there are developed two membrane bones and one cartilage bone. The cartilage bone is the =mento-meckelian= (fig. 26, A, 14), a very small ossification at the extreme anterior end. The membrane bones are the _angulo-splenial_ and the _dentary_. The _angulo-splenial_ is a strong flat bone which forms the inner and lower part of the mandible for the greater part of its length. Its dorsal surface is produced into a slight =coronoid process=. The _dentary_ (fig. 26, A, 15) is a flat plate which covers the outer surface of the anterior half of the mandible, as far forwards as the mento-meckelian. The lower jaw is devoid of teeth. The part of Meckel's cartilage which in most vertebrates ossifies, forming the articular bone, remains unossified in the Frog.

THE HYOID APPARATUS.

The =hyoid= of the adult Frog is formed of the modified hyoid and branchial arches of the tadpole. It consists of a broad thin plate of cartilage, the =basilingual plate= (fig. 29, B, 1), drawn out into two pairs of long processes, the =cornua=. The basilingual plate is broader in front than behind, and is formed from the fused ventral ends of the hyoid and branchial arches of the tadpole.

The =anterior cornua= (fig. 29, B, 2) form a pair of long slender cartilaginous rods which project from the body of the hyoid at first forwards, then backwards, and finally upwards and somewhat forwards again, to be united to the auditory capsules just below the fenestrae ovales. They are formed from the dorsal portion of the hyoid arch of the tadpole and are homologous with the cerato-hyals of the Dogfish.

The =posterior cornua= form a pair of straight bony rods diverging outwards from the posterior end of the basilingual plate. They are formed from the fourth branchial arches of the tadpole, and differ from the rest of the hyoid apparatus in being well ossified.

The =columellar chain=, which has been already described (p. 157), should be mentioned with the hyoid as it is homologous to the hyomandibular of fishes.

The =sternum= of the =Frog=, though regarded as part of the axial skeleton, is so intimately connected with the pectoral girdle, that it will be described with the appendicular skeleton.

2. THE APPENDICULAR SKELETON.

This consists of the skeleton of the two pairs of limbs and their respective girdles. It is at first entirely cartilaginous but the cartilage becomes later on mainly replaced by bone. The only bone developed in connection with the appendicular skeleton, which has no cartilaginous predecessor, is the _clavicle_.

A. THE PECTORAL GIRDLE.

This consists originally of two half rings of cartilage encircling the sides of the body a short way behind the head. These two halves meet one another in the ventral middle line, and separate the anterior elements of the sternum from the posterior ones.

Each half-ring bears on the middle of its outer and posterior surface a prominent cup, the =glenoid cavity=, with which the proximal arm-bone articulates. This cup divides the half-arch into a dorsal =scapular= and a ventral =coracoid= portion.

The =scapular portion= consists of two parts, the =suprascapula= and the =scapula=.

The =suprascapula= (fig. 30, A, 2) is a wide, thin plate attached by its ventral and narrowest border to the scapula. Its proximal and anterior half is imperfectly ossified, its whole border or sometimes only its dorsal and posterior borders consist of unaltered hyaline cartilage, while the rest of it is composed of calcified cartilage. The =scapula= (fig. 30, A, 3) is a fairly stout rod of bone constricted in the middle, and forming the dorsal half of the glenoid cavity.

The =coracoid portion= consists of three parts, the =coracoid=, =precoracoid= and _clavicle_.

The largest and most posterior of these is the _coracoid_ (fig. 30, A, 4) which like the scapula, is contracted in the middle and expanded at the ends, especially at the ventral end. It forms a large part of the glenoid cavity. The ventral ends of the coracoids which meet one another in the middle line are unossified, and form narrow strips of calcified cartilage, the =epicoracoids= (fig. 30, A, 5); these are often regarded as sternal elements.

The =precoracoid= forms a narrow strip of cartilage lying in front of the coracoid, from which it is separated by the wide =coracoid foramen= (fig. 30, A, 9). The dorsal end is continuous with an area of unossified cartilage which separates the coracoid and scapula and forms part of the glenoid cavity.

The _clavicle_ is a narrow membrane bone closely attached to the anterior surface of the precoracoid, its dorsal end is expanded.

THE STERNUM.

The sternum consists of four parts arranged in two groups; two parts to each group. The anterior members are the episternum and omosternum.

The =episternum= (fig. 30, A, 10) is a thin almost circular plate of cartilage much of which remains hyaline.

The =omosternum= (fig. 30, A, 11) is a slender bony rod widest at its posterior end; it connects the episternum with the ventral ends of the precoracoids.

The =sternum proper= is a short rod of cartilage sheathed in bone; it is contracted in the middle and expanded at each end. It bears attached to its posterior end a broad somewhat bilobed plate of partially calcified cartilage, the =xiphisternum= (fig. 30, A, 13).

B. THE ANTERIOR LIMB.

This is divisible into three parts, the =upper arm= or =brachium=, the =fore-arm= or =antibrachium=, and the =manus=.

All the larger bones have their ends formed by prominent epiphyses which do not unite with the shaft till late in life. Their articulating surfaces are covered by hyaline cartilage.

In the =upper arm= there is a single bone, the =humerus=.

This has a more or less cylindrical shaft and articulates by a prominent rounded =head= with the glenoid cavity. The distal end shows a large rounded swelling on either side of which is a =condylar ridge=, the inner or postaxial one being the larger. A prominent =deltoid ridge= runs along the proximal half of the anterior surface, and in the male frog a second equally prominent ridge runs along the distal half of the posterior surface.

The =fore-arm= consists of two bones, the =radius= and =ulna=, united together and forming the =radio-ulna=. The two bones are quite fused at their proximal ends where they form a deep cup which articulates with the distal end of the humerus, and is drawn out into a rather prominent backwardly-projecting =olecranon process=, which ossifies from a centre distinct from that of the shaft. The distal end is distinctly divided by a groove into an anterior radial and a posterior ulnar portion.

The =manus= consists of two parts, the =wrist= or =carpus= and the =hand=.

The =carpus=[57] consists of six small bones arranged in two rows. The three bones of the proximal row are the =ulnare=, =radiale= and =centrale=. The =ulnare= and =radiale= are about equal in size and articulate regularly with the radio-ulna. The =centrale= is pushed out of its normal position and lies partly on the pre-axial side, partly in front of the radiale. Of the three bones of the distal row the two pre-axial ones, =carpalia 1= and =2=, are small; carpale 2 articulates with the second metacarpal, carpale 1 with both the first and second. The third bone is large and articulates with the third, fourth and fifth metacarpals, it represents =carpalia 3-5=, with probably in addition the representative of a second centrale.

The =hand= consists of four complete digits, and a vestigial =pollex= reduced to a short metacarpal.

Each of the four complete digits consists of a =metacarpal= and a variable number of =phalanges=. The first digit, as just mentioned, has no phalanges, the second and third have two, and the fourth and fifth have three.

C. THE PELVIC GIRDLE.

The pelvic girdle of the Frog is much modified from the simple or general type found in the Newt (p. 149).

It is a V-shaped structure consisting of two halves which are fused together in the middle line posteriorly, while in front they are attached to the ends of the transverse processes of the sacral vertebra. Each half bears at its posterior end a deep cup, the =acetabulum=, with which the head of the femur articulates.

Each half of the pelvis ossifies from two centres. The anterior and upper half of the acetabulum, and the long laterally compressed bar extending forwards to meet the sacral vertebra ossify from a single centre and are generally called the =ilium=; it is probable however that they represent both the =ilium= and =pubis= of mammals[58]. The posterior part of this bone meets its fellow in a median symphysis.

The posterior third of the acetabulum is formed by a small bone, the =ischium=, which likewise meets its fellow in a median symphysis.

The ventral portion of the pelvic girdle never ossifies, even in old animals being formed only of calcified cartilage. This is generally regarded as the pubis, but it perhaps corresponds to the =acetabular bone= of mammals.

D. THE POSTERIOR LIMB.

This corresponds closely to the anterior limb and, like it, is divisible into three parts, the =thigh=, the =shin= or =crus= and the =pes=.

As was the case with the anterior limb, all the long bones have their ends formed by prominent epiphyses which do not unite with the shaft till late in life.

In the =thigh= there is only a single bone, the =femur=.

The =femur= is a moderately long, slender bone with a well-ossified hollow shaft slightly curved in a sigmoid manner. Both ends are expanded, the proximal end is hemispherical and articulates with the acetabulum, the distal end is larger and more laterally expanded.

The =shin= likewise includes a single bone, the =tibio-fibula=, but this, as can be readily seen by the grooves at the proximal and distal ends of the shaft, is formed by the fusion of two distinct bones, the =tibia= and =fibula=. The tibio-fibula is longer and straighter than the femur.

The =pes= consists of two parts, the =ankle= or =tarsus= and the =foot=.

The =tarsus= consists of two rows of structures, very different in size. The proximal row consists of two long bones, the =tibiale= and =fibulare=, which are united by common epiphyses at the two ends, while in the middle they are widely separated. The tibiale lies on the tibial or pre-axial side, and the fibulare which is the larger of the two bones on the fibular or postaxial side. The distal row of tarsals consists of three very small pieces of calcified cartilage. The postaxial of these is the largest, it articulates with the second and third metatarsals and is probably homologous with tarsalia 2 and 3 fused. The middle one is very small, it articulates with the first metatarsal and is probably tarsale 1. The pre-axial one articulates with the metatarsal of the calcar, a structure to be described immediately, and has been regarded as a =centrale=.

The =foot= includes five complete digits and a supplemental toe as well. Each of the five digits consists of a long =metatarsal= with epiphyses at both ends, and of a variable number of phalanges. The first digit or =hallux= and the second have two phalanges, the third three, the fourth, which is the largest, four, and the fifth, three. The distal phalanges have epiphyses only at their proximal ends, the others at both ends.

On the pre-axial side of the hallux is the supplemental digit, the =prehallux= or =calcar=. It consists of a short metatarsal and one or two phalanges, and is terminated distally by a horny covering of epidermal origin.

FOOTNOTES:

[55] See A. Ecker, _Die anatomie des Frosches_, Braunschweig 1864, translated by G. Haslam, Oxford, 1889, also A.M. Marshall, _The Frog_, 5th edition, Manchester and London, 1894.

[56] W.K. Parker, _Phil. Trans._ 161, 1871, p. 137, and W.K. Parker and G.T. Bettany, _The Morphology of the Skull_, London, 1877, p. 136.

[57] See G.B. Howes and W. Ridewood, _P.Z.S._, 1888, p. 141.

[58] See bottom of p. 187.