Anatomy of the Cat

Caudally there appear in the ventral view the foramen magnum (a), occipital condyles (b), and jugular processes (c). In front of the jugular processes the two tympanic bullæ (d) form prominent features, with the mastoid process (e), the stylomastoid foramen (f), and the external auditory meatus (g) on their lateral surfaces. All these structures have been described. The tympanic bullæ (d) are placed with long axes directed craniomediad, so that they converge toward their cranial ends. At the caudomedial angle of each bulla is the large jugular foramen (h), for the ninth, tenth, and eleventh nerves. Opening into the mediocaudal margin of the jugular foramen is the smaller hypoglossal foramen, for the twelfth nerve.

The craniomedial end of the tympanic bulla projects craniad as the styliform process (i). Just laterad of this process is the opening (j) into the tympanic bulla by which the tuba auditiva or Eustachian tube passes into the middle ear. A faint groove for the tube passes craniomediad from this opening, on the surface of the sphenoid. Craniolaterad of the opening for the tuba auditiva is the foramen ovale (k); craniad of this the foramen rotundum (l) is faintly indicated. On the surface of the sphenoid just craniad of the styliform process of the bulla tympani is the minute opening of the pterygoid canal (t). The orbital fissure and optic foramen are not seen in the ventral view.

The middle region of the ventral surface is narrow: it is formed by a trough-like fossa which is bounded laterally by the pterygoid processes (m) of the sphenoid and the perpendicular plates of the palatines (n). Ventrad of this lies, in the natural condition, the soft palate, converting the fossa into the nasal portion of the pharynx or nasopharynx. Craniad this fossa is bounded by the free caudal edges of the palatines; beneath which the fossa communicates with the nasal cavity by the two choanæ (o). Laterad of this median fossa are visible in the ventral view parts of the temporal and orbital fossæ, bounded laterally by the zygomatic arches (p).

The cranial part of the ventral surface is a somewhat triangular plane area formed by the palatal portions of the palatines (8), maxillaries (9), and premaxillaries (10), which together constitute the hard palate (palatum durum). Laterad and craniad this area is bounded by the alveolar borders of the maxillaries and premaxillaries bearing the teeth. The hard palate is marked near the cranial border of the palatine bones with two or more foramina which form the cranial termination of the posterior palatine canal (q). Two faint grooves pass from these foramina a short distance craniad, gradually converging: these are known as the palatine grooves (r) (sulci palatini). Near the cranial end of the hard palate are two large openings close together near the middle line: these are the foramina incisiva (or anterior palatine foramina) (s).

Cavities of the Skull

(Figs. 42 and 43).—The bones of the cranial portion of the skull enclose the cranial cavity for the brain; the facial bones enclose the nasal cavity, for the olfactory organ.

The cranial cavity is divisible into three principal fossæ: the cerebellar fossa (Fig. 43, I) caudad, for the cerebellum; the cerebral fossa (II) in the middle, for the cerebrum; the small olfactory fossa (III) at the cranial end for the olfactory bulb of the brain.

The cerebellar fossa (I) is bounded caudally by the occipital bone enclosing the foramen magnum (Fig. 42, a). Its ventral surface is formed by the basilar portion of the occipital and the petrous portions of the temporals; its lateral surface by the mastoid portions of the temporals and parts of the parietals and occipital. Its roof is formed by the parietals and interparietal. Craniad the cerebellar fossa is partly separated from the cerebral fossa by the tentorium (Fig. 42, e; Fig. 43, f) formed by the two parietals: this encloses a quadrangular opening by which the two fossæ communicate. The caudal, dorsal, and lateral walls of the cerebellar fossa are deeply marked by fossæ for the lobes of the cerebellum; the small appendicular fossa (Fig. 43, e), forming a deep indentation in the petrous bone near its dorsocaudal end, is particularly noticeable.

The following openings are found in the walls of the cerebellar fossa. Caudad is the large foramen magnum (Fig. 42, a) by which the brain-cavity communicates with the vertebral canal. Near the caudal margin of the foramen magnum, on its lateral side, just mediad of the dorsal end of the occipital condyle, is the caudal opening of the condyloid canal (Fig. 43, a) which passes craniad through the substance of the occipital bone to open just caudad of the petrous: it transmits a vein. The condyloid canal varies greatly in size in different specimens. A few millimeters craniad of the edge of the foramen magnum on the floor of the fossa is the small opening of the hypoglossal canal (Figs. 42 and 43, b), for the twelfth nerve. Just craniad of this, at the caudomedial border of the petrous, is the large jugular foramen (c). On the petrous itself, near the middle, is the internal, auditory meatus (d) divided into the dorsal facial canal for the seventh nerve, and a ventral passage for the eighth nerve. At the cranial end of the cerebellar fossa is the large opening bounded by the free edges of the tentorium.

The cerebral fossa forms much the largest part of the cranial cavity. It is bounded by the parietals (Fig. 43, 3′), squamous portions of the temporals (4), frontals (8), the sphenoid (5), and presphenoid (6). A slight rounded ridge on its lateral wall at about the position of the suture between the frontals and parietals separates a smaller cranial portion sometimes called the anterior fossa, from a larger caudal portion sometimes known as the middle fossa of the cranial cavity. The walls of the cerebral cavity are marked with numerous ridges and shallow furrows for the cerebral convolutions.

The floor of the cerebral cavity is bounded caudad by the prominent dorsum sellæ (Fig. 42, f; Fig. 43, g), just craniad of which is the rounded depression known as the sella turcica (Fig. 42, g; Fig. 43, h), for lodgment of the hypophysis. A number of foramina pierce the floor of the cavity in this region. Just ventrad of the cranial tip of the petrous portion of the temporal is the small foramen lacerum (medius). Craniad and laterad of this is a row of four foramina: the caudal one is the foramen ovale (Fig. 42, i); then come in order the foramen rotundum (j), the orbital fissure (k), and the optic foramen (Fig. 42, l; Fig. 43, k). The two optic foramina are connected by the shallow transverse chiasmatic groove (Fig. 42, m), for the optic chiasma. Another small foramen continues caudad from a groove on the floor of the orbital fissure; this opens on the ventral surface of the sphenoid, between the wing and the body of the bone. The groove and foramen constitute the pterygoid canal, which transmits a nerve,—the nerve of the pterygoid canal, or Vidian nerve.

The cranial cavity narrows at its cranial end to form the small olfactory fossa (Fig. 43, III) which lodges the olfactory bulbs. This is bounded by the frontals and the lamina cribrosa (Fig. 42, o) of the ethmoid; caudad it opens directly into the cerebral fossa. Numerous openings through the lamina cribrosa for the olfactory fibres connect the olfactory fossa with the nasal cavity. The roof of the fossa is marked by a prominent median crest from the united edges of the frontals.

The nasal cavity is almost completely filled by the ethmoid and vomer and the conchæ nasales. Its roof is formed by the nasal bones and portions of the frontals; its sides by the frontals, lachrymals, maxillaries, premaxillaries, and palatine bones; its floor by the horizontal plates of the palatines, maxillaries, and premaxillaries.

The nasal cavity opens craniad by the large nares (Fig. 39, j; Fig. 42, r), which are bounded by the premaxillary and nasal bones. In the natural condition this opening is divided by a median cartilage which is continuous with the lamina perpendicularis (Fig. 43, n) of the ethmoid, thus forming a partition which divides the nasal cavity into two separate halves. From the floor of the cranial part of the cavity rises a ridge formed of the nasal crests of the maxillaries and premaxillaries, and the cranial portion of the vomer. Farther caudad the vomer spreads out in a horizontal plane and separates from the floor of the cavity, so that the nasal cavity is thereby divided by a horizontal partition into dorsal and ventral portions. The ventral portion is small, forming the inferior meatus of the nose; it ends caudally at the choanæ (posterior nares, Fig. 41, o) which lead into the nasopharynx. That portion of the nasal cavity lying dorsad of the vomer is almost completely filled by the ethmoid and the conchæ nasales, superior and inferior. It is bounded caudally by the lamina cribrosa of the ethmoid (Fig. 42, o). The nasal cavity communicates with the cranial cavity by the foramina for the olfactory fibres in the lamina cribrosa; with the nasopharynx by the choanæ; with the exterior of the body by the nares; with the mouth-cavity by the foramina incisiva or anterior palatine foramina (Fig. 42, s); with the orbit by the sphenopalatine foramen and the nasolachrymal canal. It communicates directly also with the frontal sinuses (Fig. 43, m, m′), the sphenoidal sinuses (Fig. 43, l), and with the cells of the labyrinths of the ethmoid.

JOINTS AND LIGAMENTS OF THE SKULL.

Sutures of the Skull.—The bones of the skull join each other by means of immovable articulations known as sutures. These sutures are designated by combining the names of the bones between which they are situated: as, sphenofrontal suture (sutura sphenofrontalis), between the sphenoid and frontal; nasomaxillary suture (sutura nasomaxillaris), between the nasal and maxillary bones. When a suture joins the two corresponding bones of opposite sides the prefix inter is used, as the intermaxillary suture (sutura intermaxillaris) between the maxillaries. The sutures bounding the parietals have, however, received special names not derived in this manner. The suture caudad of the parietals, separating them from the occipital and interparietal, is known as the lambdoidal suture; that between the two parietals is the sagittal suture; that separating the parietals and squamous portions of the temporals is the squamous suture; that between the parietals and frontals is the coronal suture. The suture separating the two frontals also is known as the frontal suture, in place of interfrontal.

Articulations of the Mandible.—In man the two halves of the mandible are united craniad, so as to form a single bone. In the cat the two halves are separate, but articulate closely at the symphysis menti by a thin interarticular cartilage.

The articulation of the mandible at the mandibular fossa of the temporal is covered with a close articular capsule. The mandibular fossa is lined with cartilage. A slender ligament passes from the angular process of the mandible caudad to the external auditory meatus, being attached to the latter about 8 millimeters from its medial end. This is the stylomandibular ligament.

V. BONES OF THE THORACIC EXTREMITIES.

Scapula

(Figs. 44 and 45).—The scapula may be described as a flat triangular bone with one angle rounded. It lies beneath the muscles on the lateral face of the thorax near its cranial end. From its lateral surface there projects a flat ridge (Fig. 44, g), the spine of the scapula. The ventral end of the ridge is free as a curved process, the acromion process (Fig. 44, j).

The ventral angle of the scapula (d), the glenoid angle (lateral angle of human anatomy), is much heavier than the others and bears a concave, pear-shaped articular facet, the glenoid fossa, for articulation with the humerus. The border with which this angle is more nearly continuous may be called the glenoid border (c) (axillary border of human anatomy).

Near the narrower cranial end of the glenoid fossa is a small curved projection of the bone, the coracoid process (Fig. 45, m). The border upon which it lies is the coracoid border (b) (superior border of human anatomy). The third border is turned toward the vertebral column and is the vertebral border (a).

The angle between the glenoid and vertebral borders is the glenovertebral angle (f) (inferior angle of human anatomy), and that between the coracoid and vertebral borders the coracovertebral angle (e) (medial angle of human anatomy).

The medial or costal surface (Fig. 45) is smooth and nearly flat. A shallow furrow (n) marks the position of the spine of the scapula. Between the furrow and the coracoid border are two oblique parallel ridges (o and o′) for the insertion of muscle-fibres. Near the glenoid border is a well-marked ridge separating the subscapular fossa (III), comprising the greater part of the medial surface of the scapula, from the fossa in which the teres major muscle has origin (IV). The surface presents several nutrient foramina usually directed toward the glenoid angle.

The lateral surface (dorsal surface of human anatomy) (Fig. 44) is divided by the spine (g) into two portions. The portion of the scapula craniad of the spine and the cranial surface of the spine bound the supraspinous fossa (fossa supraspinata) (I), while the surface caudad of the spine and the caudal portion of the spine bound the infraspinous fossa (fossa infraspinata) (II).

The spine (g) begins as a triangular elevated area in the middle of the vertebral margin and runs toward the glenoid angle. It rises gradually for about two-fifths of its length and then the margin becomes broader and the spine remains of the same height to its glenoid end. There is a rough thickening, the tuberosity (h) of the spine, situated on its free border about midway between the tip of the acromion and the vertebral end of the spine. The spine is inclined toward the glenoid margin so as to form an angle of about 60 degrees with the caudal half of the lateral surface.

At the base of the acromion process (j) the margin of the spine presents a flat triangular projection, the metacromion (i), directed toward the glenoid border.

The acromion (j) continues in the direction of the spine. It is thicker than the spine, smooth and rounded on both its surfaces and both its borders, and its apex is connected by fibrous tissue to the clavicle.

The coracoid border (b) presents a slight rounded notch, the incisura scapulæ or suprascapular notch (l), just dorsad of the glenoid angle, and at its ventral end bears the coracoid process (m) which is directed ventromediad.

The glenoid angle (d) is the only one requiring special mention. Between the root of the coracoid process and the glenoid cavity it presents a tubercle, the supraglenoidal or bicipital tubercle (k), for the tendon of origin of the biceps muscle. The glenoid angle is separated by a contracted neck from the rest of the bone. Between this angle and the inner margin of the acromion there is left a deep notch, the great scapular notch.

Clavicle. Clavicula

(Fig. 46).—The clavicle in the cat is greatly reduced. It is a slender curved rod of bone imbedded in the muscles of the shoulder and connected by fibrous tissue to the apex of the acromion process. The lateral end (b) is slightly enlarged.

Humerus

(Figs. 47 and 48).—The humerus forms the support of the upper arm and articulates by its proximal end with the scapula at the glenoid cavity, and by its distal end with the radius and ulna, the bones of the lower arm. It is a nearly cylindrical bone with enlarged ends, and is so curved that its dorsal and ventral borders are hooked at the opposite ends so that it has the form of an Italic f.

The proximal end of the bone bears on its dorsomedial portion a thickening, the head of the humerus (a), which bears a smooth ovoid articular facet by which the bone articulates with the glenoid cavity of the scapula. The head is not separated from the body by a distinct anatomical neck as in the human humerus.

Along the lateral border of the proximal end of the shaft is a high rough ridge semicircular in side view, the great tuberosity (b). It gives attachment to muscles and is marked on its dorsal border by a deep depression for the tendon of the infraspinatus muscle. On the medial margin of the proximal end closely associated with the head is a smaller elevation, the lesser tuberosity (c), also for muscular attachment.

Between the greater and lesser tuberosities on the ventral surface is seen a broad groove, the sulcus intertubercularis or bicipital groove (d), which passes distad onto the surface of the shaft. In the natural state it is converted into a canal by overlying tendons and lodges the tendon of the biceps muscle.

The shaft is nearly cylindrical at its middle, but its dorsoventral diameter is slightly greater than its mediolateral diameter. Its proximal end is flattened mediolaterad, while its distal end is flattened dorsoventrad.

From the ventral margin of the greater tuberosity a ridge, the pectoral ridge (e), is continued onto the surface of the shaft, and from the dorsal margin another ridge, the deltoid ridge (f), passes distad and ventrad so as to meet the pectoral ridge near the middle of the ventral surface of the bone. On the medial margin of the bone near the junction of the first and second fourths is a roughened area (g) for the attachment of the tendons of the latissimus dorsi and teres major muscles, and on the same surface near the junction of the second and last thirds is a nutrient foramen (h).

The distal end of the bone presents a smooth saddle-shaped articular surface, which, in well-marked bones, is divided, when seen from the ventral surface, by a slight nearly median ridge into two unequal portions, lateral and medial (i and j). The lateral half is rounded and is called the capitulum (i). It is broader ventrad than dorsad, and is not continued onto the dorsal surface of the bone. It is for articulation with the proximal end of the radius.

The medial half of the surface, the trochlea (j), is concave and passes directly into the capitular surface laterad, but is limited mediad by a sharp ridge. It is continued onto the dorsal surface of the bone, where it is limited also laterad by a ridge. It articulates with the semilunar notch of the ulna.

Proximad of the trochlea the dorsal surface presents a deep fossa, the olecranon fossa, which receives the olecranon of the ulna when the arm is straightened. On the ventral surface (Fig. 47) are two shallower fossæ separated by a longitudinal ridge. The one over the trochlea receives the coronoid process of the ulna when the arm is bent, and is called thence the coronoid fossa (l). The one over the capitulum, the radial fossa (m), receives a triangular facet on the proximal end of the radius at the same time. Between the radial and coronoid fossæ on one side and the olecranon fossa on the other is only a thin plate of bone. On the medial surface of the distal end is a considerable roughened elevation, the medial epicondyle (n) (epitrochlea). It gives origin to flexor muscles and to the ulnar collateral ligaments of the elbow-joint. Opposite the medial epicondyle over the capitulum is the lateral epicondyle (o) for the origin of extensor muscles of the forearm and of the radial collateral ligaments of the elbow-joint. From the lateral epicondyle a ridge, the lateral supracondyloid ridge (p), continues proximad, curving onto the dorsal surface of the bone and ending about opposite the junction of the deltoid and pectoral ridges.

Proximad of the medial epicondyle the bone is pierced near its medial margin by an oblique oval foramen, the supracondyloid foramen (q).

Radius

(I, Figs. 49 and 50).—In the usual position the radius lies with its proximal end on the lateral side of the arm, articulating with the capitulum of the humerus. The proximal end is thus laterad of the proximal end of the ulna. Its distal end, however, lies on the medial side of the distal end of the ulna, so that the radius in the natural position crosses ventrad of the ulna.

The radius is a curved bone slightly flattened dorsoventrally, with enlarged ends. It may be described as consisting of a shaft and of a proximal and a distal end. Its proximal end presents on the ventral surface a tuberosity, the bicipital tuberosity (c), for the insertion of the tendon of the biceps muscle. Proximad of this the bone is contracted to form a neck (b) which is surmounted by a head (a). The head has on its proximal surface a depressed oval facet by which it articulates with the capitulum, and on its ulnar border a long narrow facet, the articular circumference (d), for articulation with the radial notch of the ulna; also a triangular facet (e), which fits into the radial fossa of the humerus.

The shaft is convex dorsad and concave ventrad. The distal end is somewhat pyramidal. From its medial or radial side a wedge-shaped process, the styloid process (f), extends distad. The distal surface of the end together with the lateral surface of the styloid process form a concave articular cavity (g) which fits against the scapholunar bone.

The dorsal surface of the distal end is marked by longitudinal grooves for tendons, and its lateral or ulnar surface bears a concave facet for articulation with the ulna.

Ulna

(II, Figs. 49 and 50).—The ulna is a long slender bone, flattened mediolaterad. It is enlarged at its proximal end and becomes gradually smaller toward the distal end.

The proximal end is marked ventrally by a deep excavation, the semilunar notch, or great sigmoid cavity (h). By the saddle-shaped articular surface of the semilunar notch it articulates with the trochlea. This articular surface is divided into two parts by a transverse non-articular area. The distal boundary of the semilunar notch is a blunt process, the coronoid process (i), which bears on its lateral surface a concave facet, the radial notch, for the head of the radius.

The portion of the bone proximad of the semilunar notch is called the olecranon (j). It fits into the olecranon fossa of the humerus when the arm is straightened, and is rough at its end for the insertion of tendons.

The body of the ulna becomes triangular distad. The distal end is slightly larger than the shaft just proximad of it, and bears on its radioventral side a hemispherical head for articulation with the radius. Distad of the head the bone continues as the flattened styloid process (m), which projects distad from its dorsolateral side and is smooth on the medial side of its apex, for articulation with the cuneiform bone of the wrist.

Carpus

(Fig. 51).—The carpus (wrist) consists of seven bones arranged in two rows, three in the proximal row and four in the distal row. Beginning on the medial side of the hand (thumb or radial side), the first bone in the proximal row is the scapholunar (a) (equal to the scaphoid or navicular and lunar of the human hand). It articulates with the radius. The next is the cuneiform (b), articulating with the styloid process of the ulna, and the next, which is attached to the cuneiform and projects freely ventrad, is the pisiform (c).

In the distal row the bone on the radial side is the trapezium (d); the next is the trapezoid (e), the next the os magnum (f), and the last the unciform (g). The distal row articulates with the metacarpals or bones of the palm of the hand (1-5).

In the kitten the scapholunar is represented by three bones, the scaphoid or navicular, on the radial side, the lunare, between the scaphoid and the cuneiform, and a centrale, which lies distad of the other two.

Scapholunar Bone. Os scapholunaris (Fig. 51, a).—The scapholunar is a quadrangular bone with the ventroradial angle produced into a blunt process. Its proximal surface is smooth and articulates with the distal end of the radius. The distal end is marked by oblique ridges and articulates with the unciform, os magnum, trapezoid, and trapezium. The ulnar surface articulates with the cuneiform, and the dorsal surface of the ventroradial process with the radial sesamoid.

Cuneiform Bone. (Os triquetrum BNA) (Fig. 51, b).—The cuneiform bone has the form of a flattened pyramid. Its base articulates with the unciform, its proximoulnar surface with the pisiform except at its dorsal margin, where it articulates with the styloid process of the ulna. On its proximoradial surface is a smooth facet for articulation with the scapholunar.

Pisiform Bone. Os pisiforme (Fig. 51, c).—The pisiform bone is about twice as long as broad, with enlarged ends. Its dorsal end articulates with the cuneiform, and on its proximal surface, separated from the above by a smooth ridge, is a smooth facet for articulation with the styloid process of the ulna.

Unciform Bone. (Os hamatum BNA) (Fig. 51, g).—The unciform is a wedge-shaped bone with the apex of the wedge directed proximad, and smooth for articulation with the scapholunar. By a part of its ulnar surface it articulates with the cuneiform, and by its radial surface with the os magnum. Its distal end articulates with the fourth and fifth metacarpals.

Os magnum. (Os capitatum BNA) (Fig. 51, f).—The os magnum may be described as an oblong plate bearing on its proximal surface a semicircular ridge which crosses it diagonally. The proximal end of the bone articulates with the scapholunar. Its distal end articulates with the third metacarpal except near its ventroulnar angle, where it articulates with the fourth metacarpal. Its ulnar surface articulates with the unciform. Its radial border articulates with the trapezoid, the third metacarpal, and, by two facets, with the second metacarpal.

Trapezoid. (Os multangulum minus BNA.) (Fig. 51, e).—The trapezoid is somewhat wedge-shaped, with the apex of the wedge pointing ventrad. Its proximal side articulates with the scapholunar, its distal side with the second metacarpal, its ulnar side with the os magnum, and its radial side with the trapezium.

Trapezium. (Os multangulum majus BNA.) (Fig. 51, d).—The trapezium has the form of a triangular prism curved into a semicircle. The convex face looks proximad and articulates by its ventral half with the scapholunar. The ulnar surface articulates with the second metacarpal dorsally, and ventrally with the trapezoid. Its radial surface articulates with the first metacarpal.

Bones of the Hand or Manus

(Fig. 51, 1-5).—The Metacarpals. Metacarpus.—The metacarpals are the five bones of the palm of the hand; they are numbered from one to five, beginning with the thumb. They are cylindrical elongated bones with enlarged ends. The distal end is called the head, and the proximal end the base. Each head bears a hemispherical articular facet which is marked over its ventral half by a prominent smooth ridge. The surface dorsad of the ridge articulates with the proximal head of a phalanx. The ridge and the surface at its sides are for a pair of sesamoid bones.

The first metacarpal (1) is the shortest. Its head is oblique, and it articulates by the ulnar half of its proximal surface with the trapezium (d); by the radial half with the radial sesamoid (h).

The second metacarpal (2) is marked on the proximal part of its dorsal surface by an oblique groove passing from the radial side distad to the ulnar side. The base articulates with the trapezoid (e). The ulnar surface of the proximal end articulates with the os magnum (f) and third metacarpal, while the radial surface articulates with the trapezium (d).

The third metacarpal (3) is the longest, and its base is rhomboidal with a projecting dorsoradial angle separated by a groove from the rhomboid surface. The proximal end articulates with the os magnum (f) and second metacarpal; the radial surface of the proximal end with the second, and the ulnar surface with the fourth, metacarpal.

The fourth metacarpal (4) has a base similar in form to that of the third, and when placed in position with the fifth the two form a hemispherical facet which articulates with the unciform (g) and os magnum (f). The fourth metacarpal articulates by its radial side with the third, and by its ulnar side with the fifth.

The fifth metacarpal (5) articulates by its proximal end with the unciform (g), and by the radial side of its proximal end with the fourth metacarpal.

Digits (Fig. 51).—The first digit of the hand is called the pollex (thumb), the second the index, the third the medius, the fourth the annularis, the fifth the minimus.

The first digit has two phalanges, each of the others three phalanges.

Phalanges (Fig. 51, i, j, k).—The phalanges of the proximal row (i) are elongated, flattened dorsoventrally and curved so as to be longitudinally convex dorsad. All have thickened ends. The proximal end is notched, and its proximal surface looks dorsad and is concave for the head of the metacarpal. The distal end is pulley-shaped, and the pulley surface extends farther on to the ventral than on to the dorsal surface, and serves for articulation with the middle phalanx.

The phalanges of the middle row (j) are like those of the proximal row, but shorter. The proximal surface is triangular and marked by a median facetted ridge. The whole surface is smooth and adapted to the distal end of the phalanx of the first row. The distal end is transversely elongated, so as to be cylindrical, and projects more toward the ulnar than toward the radial side. The distal phalanx (k) articulates with this cylinder so that when it is fully extended it lies on the ulnar side of the middle phalanx.

The distal phalanx (k) has the form of a quadrangular prism. It is excavated on its proximal surface for articulation with the middle phalanx. Its distal surface presents dorsad a deep excavation from the bottom of which arises a compressed plate of bone having the form of a bird’s beak. The depression receives the base of a claw, and the beak-like projection supports the claw.

Sesamoid Bones of the Hand. Ossa sesamoidea.—The hand contains, in addition to those already described, eleven small bones that are developed in tendons.

One of these, the radial sesamoid (Fig. 51, h), is closely applied to the radial end of the scapholunar bone. It is developed in the tendon of the extensor brevis pollicis muscle.

The other ten occur in pairs as small flattened curved bones on the ventral side of the joint between each metacarpal and the phalanx with which it articulates.

JOINTS AND LIGAMENTS OF THE THORACIC LIMBS.

The shoulder-joint is an arthrodial or ball-and-socket joint. The bones entering into its formation are the scapula and the humerus.

The capsular ligament or articular capsule is very ample and allows for extended movement of the humerus. It is attached to the edge of the glenoid fossa of the scapula and passing distad covers the head of the humerus and is inserted at the line of junction of the shaft and the epiphysis which forms the head of the bone. On the lateral side of the ventral surface of the humerus the attachment continues distad about two centimeters along the lateral edge of the bicipital groove. On the medial side the insertion passes over the proximal end of the lesser tuberosity. A strong transverse band passes from the greater tuberosity to the lesser tuberosity and bridges the bicipital groove, converting it into a canal. The lateral and medial parts of the capsule are strengthened by thicker bands of fibres, the more prominent medial one of which passes from the coracoid process of the scapula to the lesser tuberosity. To the capsule are closely united parts of the supraspinatus, infraspinatus, coracobrachialis, and subscapularis muscles. A synovial membrane lines the capsule within and forms a sheath around the biceps tendon, so that the latter does not actually enter the synovial capsule.

The elbow-joint (Figs. 52 and 53) is a ginglymus or hinge-joint. The bones which enter into it are the humerus, radius, and ulna.

The capsule of the joint forms a sac, with the following attachments to the bones: (1) To the humerus it is attached at the proximal edge of the coronoid and radial fossæ; to the sides of the capitulum and trochlea distad of the two epicondyles, and to the distal edge of the olecranon fossa. (2) To the ulna it is attached at the edges of the radial and semilunar notches; (3) to the radius around the articular facet, two or three centimeters distad of the border. Many of the muscles of this region are closely attached to the capsule.