Part 17

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.

FIG. 53.--ELBOW-JOINT, LATERAL VIEW.

Fig. 52.--1, humerus; 2, ulna; 3, radius. a and b, the two medial collateral ligaments.

Fig. 53.--1, humerus; 2, radius; 3, ulna. a, dorsal collateral ligament; b, ventral collateral ligament; c, annular ligament.]

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.

=The Wrist.=--At the wrist or carpus there are in reality three joints, the first between the radius and ulna proximad and the first row of carpals distad, the second between the two rows of carpal bones, the third between the distal row of carpals and the metacarpals. The first two are movable joints; the third is not. Each of these three joints has a capsule, and the bones entering into the joints are interconnected by numerous ligaments. These ligaments are named by combining the names of the two bones which they interconnect. Ligaments which interconnect bones of the same row in the carpus are sometimes distinguished as =interosseous= ligaments, as contrasted with =intercarpal= ligaments, which connect together bones of different rows. According to their position the ligaments may also be distinguished as dorsal, ventral, and lateral. Detailed descriptions and figures of all these ligaments are given by Strauss-Durckheim.

=Metacarpals.=--The joint between the carpals and metacarpals has been described. At the distal end of the metacarpals the articulations with the phalanges have each a capsule. The joint is further strengthened by a double lateral ligament on each side. The two sesamoid bones at each joint are interconnected by a strong transverse ligament, and each is connected with the head of the metacarpal and the base of the first phalanx by a lateral ligament.

=Phalanges.=--Between the phalanges the joints possess capsules, and each has a radial and an ulnar lateral ligament.

VI. BONES OF THE PELVIC EXTREMITIES.

=Innominate Bones. Ossa innominata.= (=Os coxæ BNA=) (Figs. 54 and 55).--The two innominate bones articulate with the sacrum and extend thence caudoventrad and finally turn mediad and unite in the middle line, forming the =symphysis pubis=. They thus form an arch, =the pelvic arch=, =pelvic girdle= or =pelvis=, which is closed dorsad by the sacrum.

In the middle of the lateral surface of each bone is a hemispherical depression, the =acetabulum= (Fig. 55, d), which receives the head of the femur.

I, ilium; II, ischium; III, pubis; IV, acetabular bone. a, acetabulum; b, obturator foramen.]

In the kitten each innominate bone is composed of three principal parts united by sutures (Fig. 54). From the sacrum to the acetabulum is a single bar, the =ilium= (I). Caudad of the acetabulum are two bars. The dorsal one of these is the =ischium= (II), and the ventral one is the =pubis= (III). The ischium enters into the formation of the acetabulum (a), but the pubis does not. The two bones, however, are in contact at the ventral edge of the acetabulum. From this point they diverge, but unite with one another again near the middle line and thus enclose an oval foramen, the =obturator foramen= (b). Wedged between the ilium, ischium, and pubis at their point of junction and helping to form the acetabulum is a small irregular bone, the =acetabular bone= (IV). In the adult cat these four parts are united into a single bone which is nevertheless usually described, ignoring the acetabular piece, as made up of ilium (Fig. 55, I), ischium (II), and pubis (III).

I, ilium; II, ischium; III, pubis. a, crest of the ilium; b, posterior inferior spine; c, great sciatic notch; d, acetabulum; d′, incisura acetabuli; e, spine of the ischium; f, lesser sciatic notch; g, tuberosity of the ischium; h, ramus of the ischium; i, ramus of the pubis; j, obturator foramen; k, pubic tubercle; l, iliopectineal line; l′, ilio-pectineal eminence; m, anterior superior process.]

The =ilium= (I) is somewhat contracted at the middle and broader at its ends. One end enters into the acetabulum (Fig. 55, d) and forms about one-fifth the articular surface. This end is also the thickest part of the bone. The lateral surface of the ilium is concave for the attachment of muscles. The medial surface is smooth over its acetabular half and rough over its sacral half. The rough portion is marked at its junction with the smooth portion by the ear-shaped =auricular impression= by which the bone articulates directly with the sacrum. The caudal half of that part of the medial surface craniad of the auricular impression gives attachment to the ilio-sacral ligaments which bind the ilium to the sacrum. The dorsal border is straight at its cranial end and concave and rounded at its caudal end. Between the two portions and at the dorsal edge of the auricular surface is a protuberance corresponding to the =posterior inferior spine= (Fig. 55, b) of the human ilium. The concavity of the dorsal border (c) corresponds to the great sciatic notch of the human ilium. At its caudal end is the short =spine of the ischium= (e), which is not a part of the ilium. The ventral border of the ilium is broad caudad, becoming narrower craniad. The lateral margin of the ventral border is continued to a tuberosity at the edge of the acetabulum; its medial margin is called the =iliopectineal line= (l) and extends on the pubis to the symphysis. An eminence, the =iliopectineal eminence= (l′), on the iliopectineal line, lies opposite to the acetabulum at about the junction of the ilium and pubis. The cranial end of the bone is thickened, forming the =crest= (a) of the ilium. At the junction of the crest with the ventral border is a projection, the =anterior superior process= (m) of human anatomy.

The =pubis= (III) (including the acetabular bone) enters into the formation of the acetabulum (d) constituting about one-sixth the circumference, but less than one-sixth its area. It may be described as a flat, curved bone, contracted at the middle and expanded at the ends. The dorsal end enters into the acetabulum; the ventral end unites with the opposite bone at the symphysis pubis and sends caudad a projection, the =ramus= (i) of the pubis, which unites with the ramus of the opposite bone to form about two-thirds of the entire symphysis. At the sides of the symphysis a slightly marked angle projects craniad from each of the pubic bones; these two together constitute the =pubic tubercle= (k), for the origin of the rectus abdominis muscle. The surfaces of the ramus are smooth. One of its borders is concave and enters into the formation of the obturator foramen (j). Another of its borders is the iliopectineal line (l). Its third border is rough for the symphysis.

The =ischium= (II) has the form of a triangular prism contracted at the middle. Its cranial end forms nearly two-thirds of the acetabulum. Its caudal end bears dorsad a rough thickening, the =tuberosity= of the ischium (g). From the caudal half of the ventral border of the bone a sickle-shaped process, the =ramus= (h) of the ischium, curves medioventrad and then craniad and joins the ramus of the pubis. Its medial border is rough and enters into the symphysis, forming the caudal one-third. The lateral angle of the bone is rounded. Its dorsal angle is marked near the cranial end by the =spine= (e) of the ischium. The concavity between this spine and the tuberosity corresponds to the =lesser sciatic notch= (f) of human anatomy.

The acetabulum (d) is cup-shaped. The ventral one-sixth of its border is deficient and a broad groove extends from the deficiency to the bottom of the cup. The deficiency, =incisura acetabuli=, or =acetabular notch= (d′), is closed naturally by a ligament, and the groove gives origin to the ligament (ligamentum teres) which attaches the head of the femur.