Part 15

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=.

a, medial end; b, lateral end.]

=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.

FIG. 48.--HUMERUS, MEDIAL SIDE.

a, head; b, greater tuberosity; c, lesser tuberosity; d, bicipital groove; e, pectoral ridge; f, deltoid ridge; g, rough area for insertion of latissimus dorsi and teres major; h, nutrient foramen; i, capitulum; j, trochlea; l, coronoid fossa; m, radial fossa; n, medial epicondyle; o, lateral epicondyle; q, supracondyloid foramen.]

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.

FIG. 50.--RADIUS AND ULNA, VENTROMEDIAL VIEW.