Part 8

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.

Closely connected with the capsule of the joint are the =collateral ligaments=. The two =medial= collateral ligaments (Fig. 52) arise from the medial epicondyle. One (_b_) passes distad and laterad to the interval between the radius and ulna; here it divides, one branch going to the head of the radius, while the other is attached to the lateral surface of the ulna at the edge of the semilunar notch. The second medial ligament (_a_) lies dorsad of the first; it passes to the medial surface of the ulna, at the distal edge of the semilunar notch.

The two collateral ligaments on the =lateral= side (Fig. 53) arise from the lateral epicondyle. The ventral one (_b_) passes almost directly distad and is inserted into the lateral surface of the proximal end of the radius about one centimeter from the articular surface; its inner surface is partly united to the annular ligament (_c_) of the radius. The dorsal one (_a_) is attached to the lateral border of the semilunar notch of the ulna.

=Articulations of Radius and Ulna.=--The proximal =radio-ulnar articulation= (Fig. 53) is by a pivot-joint or trochoid. The two bones are held in place by the =annular= ligament (Fig. 53, _c_). This is attached on the lateral side to the dorsal border of the radial notch of the ulna, passes around the head of the radius, receiving some ligamentous fibres which come from the lateral epicondyle, and is attached to the coronoid process of the ulna. The annular ligament is closely united with the capsule of the joint.

The radius and ulna are united for about their middle third by the thin =interosseous membrane=, which fills the interosseous space between their adjacent edges.