Part 3

Sometimes there are at first two centers for the body which soon fuse. The process of ossification extends from the lateral centers to form not only the corresponding part of the arch, but also the processes and a part of the body next to the root of the arch (Radix arcus). In the horse and ox the body and arch are usually fused at birth, but the epiphyses do not fuse till growth is complete. In the pig, sheep, and dog the body and arch are united at birth by cartilage (neurocentral synchondrosis), but fuse in the first few months.

THE RIBS

The ribs (Costæ) are elongated curved bones which form the skeleton of the lateral thoracic walls. They are arranged serially in pairs which usually correspond in number to the thoracic vertebræ. Each articulates dorsally with the spine and is continued ventrally by a =costal cartilage=. Those which articulate with the sternum by means of their cartilages are termed =sternal= or =“true” ribs= (Costæ sternales s. veræ); the remainder are =asternal= or =“false” ribs= (Costæ asternales s. spuriæ). Ribs at the end of the series which have their ventral ends free in the abdominal wall are named =floating ribs= (Costæ fluctuantes). The intervals between the ribs are termed =intercostal spaces= (Spatia intercostalia).

A typical rib[4] consists of a =shaft= and two =extremities=. The =shaft= (Corpus costæ) is band-like and varies much in length, breadth, and curvature. In the case of some ribs the curvature is not uniform, but is most accentuated at a certain point, termed the =angle of the rib= (Angulus costæ); this occurs at a variable distance from the vertebral end, and is usually marked by a rough ridge. The direction also varies; the first rib is almost vertical, while the remainder slope backward in increasing degree. The =external= surface is convex, and the =internal= flattened from edge to edge; on the latter, close to the posterior border, is the =costal groove= (Sulcus costalis), which fades out ventrally. It contains the intercostal vein. The =anterior= and =posterior borders= are thin and sharp on some ribs, rounded on others.

The =vertebral extremity= (Extremitas vertebralis) consists of the =head=, =neck=, and =tubercle=. The =head= (Capitulum costæ) is the actual end of the rib, and is rounded and somewhat enlarged. It presents two facets (Facies articularis capituli costæ) for articulation with the bodies of two adjacent thoracic vertebræ; these surfaces are separated by a groove in which the conjugal ligament is attached. The =neck= (Collum costæ) joins the head to the shaft. It varies in length and diameter. Its outer surface is rough, its inner smooth. The =tubercle= (Tuberculum costæ) projects backward at the junction of the neck and shaft. It has a facet (Facies articularis tuberculi costæ) for articulation with the transverse process of the posterior vertebra of the two with which the head articulates. The tubercle gradually approaches the head in the posterior ribs, and eventually fuses with it.

The =sternal extremity= (Extremitas sternalis) is commonly slightly enlarged, and has a rough depression in which the costal cartilage is embedded.

=Development.=—The ribs are ossified in cartilage from three centers—one each for the shaft (and sternal end), head, and tubercle; the third center does not occur in the last two ribs.

THE COSTAL CARTILAGES

These (Cartilagines costales) are bars of hyaline cartilage which continue the ribs. Those of the sternal ribs articulate with the sternum, while the remainder overlap and are attached to each other to form the =costal arch= (Arcus costalis).

THE STERNUM

The =sternum= or breast-bone is a median segmental bone which completes the skeleton of the thorax ventrally, and articulates with the cartilages of the sternal ribs laterally. It consists of six to eight bony segments (Sternebræ) connected by intervening cartilage in the young subject. Its form varies with that of the thorax in general and with the development of the clavicles in animals in which they are present. Its anterior extremity, the =manubrium sterni= or =presternum=, is specially affected by the latter factor, being broad and strong when the clavicles are well developed and articulate with it (as in man), relatively small and laterally compressed when they are absent (as in the horse) or rudimentary (as in the dog). The cartilages of the first pair of ribs articulate with it. The =body= or =mesosternum= (Corpus sterni) presents laterally, at the junction of the segments, concave facets (Incisuræ costales) for articulation with the cartilages of the sternal ribs. The posterior extremity or =metasternum= presents the =xiphoid= (or ensiform) =cartilage= (Processus xiphoideus); this is thin and plate-like, as in the horse and ox, or narrow and short, as in the pig and dog.

=Development.=—The cartilaginous sternum is formed by the fusion medially of two lateral bars which unite the ventral ends of the first eight or nine costal cartilages, and is primitively unsegmented. The manubrium ossifies from a single center, but the centers for the other segments appear to be primitively paired. The sternum never becomes completely ossified; details in regard to persisting cartilage will be given in the special descriptions. The layer of compact tissue is for the greater part very thin and the spongy substance is open-meshed and very vascular.

THE THORAX

The skeleton of the thorax comprises the thoracic vertebræ dorsally, the ribs and their cartilages laterally, and the sternum ventrally. The =thoracic cavity= (Cavum thoracis) resembles in shape an irregular truncated cone; it is compressed laterally, especially in front, and the dorsal wall or roof is much longer than the ventral wall or floor. The =anterior aperture= (Apertura thoracis cranialis) is bounded by the first thoracic vertebra dorsally, the first pair of ribs and their cartilages laterally, and the manubrium sterni ventrally. The =posterior aperture= (Apertura thoracis caudalis) is bounded by the last thoracic vertebra, the last pair of ribs, the costal arches, and the anterior part of the xiphoid cartilage.

It may be noted here that the diaphragm (which forms the partition between the thoracic and abdominal cavities) does not follow the costal arches in its posterior attachment, so that the posterior ribs enter also into the formation of the abdominal wall.

THE SKULL

The term skull is usually understood to include all of the bones of the head. The head consists of the cranium and the face, and it is therefore convenient to divide the bones into cranial and facial groups.

The =cranial bones= (Ossa cranii) inclose the brain with its membranes and vessels and the essential organs of hearing. They concur with the facial bones in forming the orbital and nasal cavities, in which the peripheral organs of sight and of smell are situated.

The =facial bones= (Ossa faciei) form the skeleton of the oral and nasal cavities, and also support the larynx and the root of the tongue.

Most of the bones of the skull are flat bones, developed in membrane; those of the cranial base may be classed as irregular, and are cartilage bones. Only two form permanent movable joints with other parts of the skull. The mandible or lower jaw-bone forms diarthrodial joints with the temporal bones, and the hyoid bone is attached to the latter by bars of cartilage. The other bones form immovable joints, most of which disappear with age.

In order to study the separate bones, skulls of young subjects are necessary, since later most of the lines of demarcation become effaced. The relations of each bone to its surroundings should be specially noted, since the final object is to understand the skull as a whole. In the descriptions which follow the skull is considered with its long axis horizontal, and that of the horse will serve as a type.

THE BONES OF THE THORACIC LIMB

The thoracic limb consists of four chief segments, viz., the shoulder girdle, the arm, the forearm, and the forefoot or manus.

The =shoulder girdle= (Cingulum extremitatis thoracicæ), when fully developed, consists of three bones—the =scapula= or shoulder-blade, the =coracoid=, and the =clavicle= or collar-bone. In the domesticated mammals only the scapula, a large, flat bone, is well developed, and the small coracoid element has fused with it, while the clavicle is either absent or is a small rudiment embedded in the mastoido-humeralis muscle. There is therefore no articulation of the shoulder with the axial skeleton.

The shoulder girdle is fully developed in birds and the lower mammals (monotremata). In the higher mammals the coracoid is reduced to the coracoid process of the scapula, and the development of the clavicle is in conformity with the function of the limb. Thus in typical quadrupeds, such as the horse and ox, in which the forelimbs are used only for support and locomotion, the clavicle is absent. Other animals which use these limbs for grasping, burrowing, climbing, etc. (_e.g._, man, apes, moles), have well-developed clavicles which connect the scapula with the sternum.

The =arm= (Brachium) contains a single long bone, the humerus or arm bone.

In the =forearm= (Antibrachium) are two long bones, the =radius= and =ulna=. These vary in relative size and mobility. In the horse and ox the two bones are fused, and the lower part of the limb is fixed in the position of pronation. The radius is placed in front and supports the weight. The ulna is well developed only in its upper part, which forms a lever for the extensor muscles of the elbow. In the pig the ulna is the larger and longer of the two bones, but is closely attached to the back of the radius. In the dog the ulna is also well developed and a small amount of movement is possible between the two bones.

The =forefoot= or =hand= (Manus) consists of three subdivisions, viz., the =carpus=, =metacarpus=, and =digit= or =digits=.

The =carpus=, popularly termed the “knee” in animals, and homologous with the wrist of man, contains a group of short bones (Ossa carpi). These are typically eight in number and are arranged in two transverse rows—a proximal or antibrachial, and a distal or metacarpal. The bones of the proximal row, named from the radial to the ulnar side (_i. e._, from within outward), are the =radial=, =intermediate=, =ulnar=, and =accessory= carpal bones. The bones of the distal row are designated numerically, in the same direction, as =first=, =second=, =third=, and =fourth= carpal bones.

This nomenclature, introduced by Gegenbaur, and now used largely by comparative anatomists, seems decidedly preferable to the variety of terms borrowed from human anatomy and based on the form of the bones in man. The following table of synonyms in common use is appended for comparison. The Latin terms and abbreviated notations are given in parenthesis.

Radial (Os carpi radiale, Cr) Scaphoid Intermediate (Os carpi intermedium, Ci) Semilunar Ulnar (Os carpi ulnare, Cu) Cuneiform Accessory (Os carpi accessorium, Ca) Pisiform First carpal (Os carpale primum, C1) Trapezium Second carpal (Os carpale secundum, C2) Trapezoid Third carpal (Os carpale tertium, C3) Os magnum Fourth carpal (Os carpale quartum, C4) Unciform

The central carpal bone (Os carpi centrale) is omitted, since it is not a separate element in the animals under consideration here.

The =metacarpus= contains typically five =metacarpal bones= (Ossa metacarpalia I–V), one for each digit; they are long bones and are designated numerically from within outward. This arrangement occurs in the dog, although the first metacarpal is much smaller than the others, and the second and fifth are somewhat reduced. Further reduction has taken place in the other animals, resulting in the perissodactyl and artiodactyl forms. In the horse the first and fifth metacarpals are absent, the third is the large supporting metacarpal bone and carries the single digit, while the second and fourth are much reduced. In artiodactyls (_e. g._, ox, sheep, pig) the third and fourth are the chief metacarpals and carry the well developed digits; they are fused in the ox and sheep. The others are variously reduced or absent as noted in the special descriptions to follow.

The fossil remains of the ancestors of the existing Equidæ illustrate in a most complete manner the reduction which has occurred in this respect. The earliest known ancestor of the horse, Eohippus or Hyracotherium of the Lower Eocene, had four well developed metacarpal bones, each of which carried a digit; the first metacarpal bone was small. Eleven intermediate stages show the gradual evolution of the race from this primitive animal, which was about the size of the domestic cat. There is reason to believe that earlier forms had five digits.

The =digits= (Digiti manus) are homologous with the fingers of man, and are typically five in number. They are designated numerically from the radial to the ulnar side, in correspondence with the metacarpus. The full number is present in the dog. In the ox and pig the third and fourth are well developed and support the weight, while the second and fifth are reduced. The existing horse has a single digit, the third of his polydactyl ancestors. The skeleton of each fully developed digit consists of three =phalanges= and certain =sesamoid bones=. The =first= or =proximal phalanx= (Phalanx prima) articulates with the corresponding metacarpal bone above and with the =second= or =middle phalanx= (Phalanx secunda) below. The =third= or =distal phalanx= (Phalanx tertia) is inclosed in the hoof or claw, and is modified to conform to the latter. The =sesamoid bones= (Ossa sesamoidea) are developed along the course of the flexor tendons or in the joint capsules. Two =proximal sesamoids= (Ossa sesamoidea phalangis primæ) occur at the flexor side of the metacarpo-phalangeal joint and form a pulley for the flexor tendon. The =distal sesamoids= (Ossa sesamoidea phalangis tertiæ) are similarly placed between the deep flexor tendon and the joint between the second and third phalanx; they are absent in the dog, which has a small sesamoid on the extensor side of the metacarpo-phalangeal joints, and often at the proximal interphalangeal joint also.

Numerous cases are recorded of the occurrence of supernumerary digits (hyperdactylism) in the horse and other animals. In some pigs, on the other hand, the two chief digits are fused, and the condition appears to be inherited.

THE BONES OF THE PELVIC LIMB

The pelvic limb, like the thoracic, consists of four segments, viz., the pelvic girdle, thigh, leg, and the hind foot or pes; the last is subdivided into tarsus, metatarsus, and digits.

The =pelvic girdle= (Cingulum extremitatis pelvinæ) consists of the =os coxæ= or hip bone, which joins its fellow of the opposite side ventrally at the symphysis pelvis, and articulates very firmly with the sacrum dorsally. The two coxal bones, together with the sacrum and the first two or three coccygeal vertebræ, constitute the bony pelvis. The os coxæ consists originally of three flat bones, the =ilium=, =ischium=, and =pubis=, which meet at the acetabulum, a large cotyloid cavity that articulates with the head of the femur. These three parts are fused before growth is complete, but are considered separately for convenience of description. The =ilium= (Os ilium) is situated in the lateral wall of the pelvis, the =pubis= (Os pubis) in the anterior part, and the =ischium= (Os ischii) in the posterior part of the ventral wall.

The =thigh= (Femur), like the arm, contains a single large, long bone, the =femur= or thigh bone (Os femoris). This articulates with the acetabulum above and the tibia and patella below.

The skeleton of the leg (Crus) comprises three bones (Ossa cruris), viz., the =tibia=, =fibula=, and =patella=. The =tibia= is a large prismatic long bone which supports the weight, and articulates below with the tibial tarsal bone. The =fibula= is situated along the outer border of the tibia, from which it is separated by the =interosseous space= of the leg. It is much more slender than the tibia and does not articulate with the femur. In the pig and dog it has a complete shaft and two extremities, but in the horse and ox it is much reduced and otherwise modified. The =patella= or “knee-cap” is a short bone which articulates with the trochlea of the distal end of the femur; it is to be regarded as a large sesamoid bone intercalated in the tendon of the quadriceps femoris muscle.

The =tarsus= or “hock” consists of a group of short bones (Ossa tarsi) numbering five to seven in the different animals. The proximal or crural row consists of two bones, the =tibial= and =fibular tarsals=; the former is situated at the inner or tibial side, and has a trochlea for articulation with the distal end of the tibia; the latter, situated externally, has a process, the =tuber calcis=, which projects upward and backward and constitutes a lever for the muscles which extend the hock joint. The distal or metatarsal row consists of four bones when seven tarsal elements are present, as in the pig and dog. They are best designated numerically as =first tarsal=, =second tarsal=, etc. The =central tarsal= is interposed between the rows.

The preceding terms are anglicized abbreviations of those introduced by Gegenbaur into comparative anatomy. The Latin names and synonyms are given in the following table.

Tibial (Os tarsi tibiale, Tt.) Astragalus or Talus Fibular (Os tarsi fibulare, Tf.) Calcaneus or Os calcis Central (Os tarsi centrale, Tc.) Scaphoid or Navicular First Tarsal (Os tarsale primum, T1) First or internal cuneiform Second Tarsal (Os tarsale secundum, T2) Second or middle cuneiform Third Tarsal (Os tarsale tertium, T3) Third or external cuneiform Fourth Tarsal (Os tarsale quartum, T4) Cuboid.

The =metatarsal= and =digital bones= resemble in general those of the corresponding regions of the thoracic limb; the differential features will be noted in the special descriptions.

THE SKELETON OF THE HORSE

The skeleton of the horse consists of 205 bones, as shown in the following table.

Vertebral column 54 Ribs 36 Sternum 1 Skull (including auditory ossicles) 34 Thoracic limbs 40 Pelvic limbs 40 ———— 205

In this enumeration the average number of coccygeal vertebræ is taken to be 18, the temporal and os coxæ are not divided into parts, the usual number of carpal and tarsal elements is taken, and the sesamoids are included.

THE VERTEBRAL COLUMN

The vertebral formula of the horse is C_{7}T_{18}L_{6}S_{5}Cy_{15–21}.

THE CERVICAL VERTEBRÆ

These are quadrangular, massive, and longer than the vertebræ of other regions; they decrease in length from the second to the last. The =third=, =fourth=, and =fifth= are typical, and have the following characters:

1. The =bodies= are long as compared with those of other vertebræ. Each presents a median =ventral spine= or =crest=, which becomes more prominent as it is traced backward, and is tuberculate at its posterior end. The =lateral aspect= is concave. The =dorsal surface= has a flat central area which is narrow in the middle of the vertebræ, and wide at either end; it gives attachment to the superior common ligament. On either side of this area is a groove which lodges the longitudinal spinal vein. These lateral grooves are connected at the middle of the surface by a transverse furrow, in which there are several foramina through which veins emerge from the spongy substance of the body. The =anterior extremity= presents a head which has an oval articular surface, strongly convex, and wider above than below. The =posterior extremity= is larger and has a nearly circular cotyloid cavity.

2. The =arches= are large and strong. They are perforated on either side by a foramen which communicates with the foramen transversarium. The vertebral notches are large.

3. The =articular processes= are large. Their articular surfaces are extensive, oval in outline, and slightly concave; the anterior ones are directed upward and inward, the posterior downward and outward. The remaining surface is mainly roughened for ligamentous and muscular attachment. A crest connects the articular processes of the same side on the fourth and fifth; on the third it does not reach the anterior process.

4. The =transverse processes= are large and plate-like. Each arises by two roots, one from the arch and one from the body; between these is the =foramen transversarium=, through which the vertebral artery passes. The process divides externally into anterior and posterior branches, which are thickened and rough for muscular attachment.

5. The =spinous process= is represented by a crest (Crista spinosa), which widens behind, and is connected by ridges with the posterior articular processes.

The =sixth= cervical vertebra has the following distinctive features: It is shorter and wider than the fifth. The =arch= is large, especially posteriorly. The =posterior articular= processes are shorter, thicker and further apart; they are connected with the anterior ones by a thick ridge. The =spinous process= is less rudimentary; it is half an inch or more (ca. 1.5 cm.) in height. The =transverse processes= have three branches; the third part is a thick, almost sagittal plate, which forms with its fellow and the body a wide ventral groove on the posterior part of the vertebra; the other branches correspond to those of the typical vertebræ, but are short and thicker. The =foramen transversarium= is large; below its posterior end is a fossa. The =ventral crest= is small and is less prominent posteriorly.

The third branch of the transverse process and the fossa are sometimes absent on one side.