Part 7

From the posterior edge of the second, the third, the fourth, and the fifth ribs, and near the middle of the dorsal segment, are flat, uncinate, bony processes which project upward and backward, overlying in each case, the succeeding rib and giving greater surface for muscular attachments and greater stability to the thorax.

=The Sternum= (Fig. 4, No. 40; Fig. 10, No. A, 9). Location.—The sternum, or breast-bone forms the inferior portion, or floor of the thoracic cavity.

Description.—The sternum is a quadrilateral, curved plate with processes projecting from each angle and from the middle of the anterior and posterior borders. The posterior medial projection, or metasternum, is the longest, and has a tall, plate-like ridge—the sternal crest, crista sterni—running along its ventral surface. The crest serves the important function of increasing the bony area for the attachment of the powerful muscles which move the wings. The anterior medial projection, or rostrum, is short, and pierced at its root by an opening from which extend two elongated, saddle-shaped depressions into which the end of the coracoid bones are received.

The plate-like process of bone, the posterior lateral process, which projects from the caudal angles of the sternum soon divides into two parts. The shorter of these, the oblique process, broadens toward its free end and covers the sternal segments of the last two ribs. The sides of the sternum are thus provided with an external and an internal process forming an external and an internal notch. These notches are bridged over by a broad ligament, to which the muscles are attached. In poor flyers, as the domestic fowl, these notches are large. The posterior end of the sternum is called the xiphoid process or processus xiphoideus. Anteriorly the sternum is provided with lateral external processes, the costal processes. The lateral borders of the sternum are pitted by four depressions into which the sternal segments of the ribs are received. The dorsal, or inner surface of the bone is pierced by openings by which the air-sacs communicate with the interior.

THE APPENDICULAR SKELETON

The appendicular skeleton consists of the shoulder girdle, the fore limb, the pelvic girdle, or hip bone, and the hind limb. The shoulder girdle consists of the scapula, the coracoid, and the clavicle. The fore limb consists of the arm, forearm, and the hand. The arm consists of the humerus, the forearm of the radius and the ulna; and the hand consists of the carpus, the metacarpus, and the phalanges. The pelvic girdle consists of the ilium, the ischium, and the pubis. The hind limb consists of the thigh, the leg, and the foot. The thigh consists of the femur, and the leg consists of the tibia and the fibula. The foot consists of the metatarsus and the phalanges.

THE SHOULDER GIRDLE

The shoulder girdle consists of the scapula, the clavicle and the coracoid.

=The Scapula.= Location.—The scapula (Fig. 11, A, B) lies on the outer and superior rib surface, extending parallel with the dorsal vertebræ.

Description.—The scapula is a thin, sword-like bone, becoming thicker as it approaches the shoulder-joint. The scapula expands and becomes thin near the free end, which reaches nearly to the antero-lateral portion of the ilium (Fig. 4, No. 24). The scapula articulates with the coracoid. A pneumatic foramen is located at the base of the acromion process. The anterior part of the scapula is provided with an articular head and is provided with an inner process, called the processus furcularis, which lies near the furcula and coracoid. An outer stronger processus humeralis forms the posterior half of the glenoid cavity, or cavitas glenoidalis, in which the humerus articulates and the processus coracoideus.

=The Clavicle.= Location.—The clavicles, commonly known as the wish bone, are located in the anterior chest region.

Description.—The clavicles are long, slender bones uniting below in the hypocledium, a laterally flattened process. They are joined to the upper end of the coracoid by fibrous cartilage. The hypocledium is joined to the anterior point of the sternum by the claviculosternal ligament. The clavicles, as united by the hypocledium, form a v-shaped structure called the furculum, or, popularly, the wish bone.

The forks play the part of an elastic spring, whose office it is to prevent the wings from coming toward each other during contraction of the depressor muscles. The conformation of this bone is, therefore, like the sternum, related to the extent and power of flight; and for this reason it is that, in swift flyers, the two branches of the furculum are thick, solid, widely separated, and curved like a U; while in those that fly heavily and with difficulty, these branches are thin and weak, and join at an acute angle. The latter formation greatly diminishes its strength, and lessens, in a singular manner, the reactionary power of the bony arch it represents.

=The Coracoid.= Location.—The coracoid is located just back of the clavicle and at the side of the entrance of the thoracic cavity. It is the strongest bone of the shoulder girdle, extending upward, outward, and forward. It articulates inferiorly with the sternum and superiorly with the humerus and the scapula, and is attached to the superior end of the furcular limb by a fibrous cartilage.

Description.—It is thinnest in the center or shaft and broadens toward the inferior extremity. The upper hook-like part of the coracoid forms the fore part of the glenoid cavity, and together with the scapula and furcula form the foramen triosseum through which passes the tendon of the elevator muscle of the wing. The upper end flattens out into three tuberosities, the tuberositas furcularis which is thick and to which attaches the limb of the furcula, the tuberositas scapularis which unites to the scapula, and the tuberositas humeralis which lies between these and articulates with the humerus.

THE FORE LIMB

The bones of the fore limb are humerus, ulna, radius, carpus, metacarpus, and phalanges.

=The Humerus.= Location.—The humerus constitutes the arm, which, when at rest, lies parallel to the dorsal vertebræ. It articulates superiorly with the glenoid cavity, a shallow ball-and-socket joint, formed by the scapula and the coracoid; and inferiorly with the ulna and radius.

Description.—The proximal extremity of the humerus is provided with a trochanter (Fig. 11, G, No. 5) and a large oval head or caput humeri, which articulates in the glenoid cavity. The head is an elongate, semi-oval convexity, with the long axis transverse from the radial to the ulnar side and with the ends continued into upper and lower crests. The upper crest of the head of the humerus is on the radial side and the lower crest on the under side. Under this latter crest there is a pneumatic fossa (Fig. 11, F, No. 1), at the upper end of which there is an opening into the bone, the pneumatic foramen, which brings the air-sac into communication with the air space of the bone. The shaft, or corpus humeris, is irregularly cylindrical and slightly S-shaped.

The proximal part of the shaft, which is expanded on the palmar side, is concave across and convex lengthwise. The distal part is slightly flattened. The shaft of the humerus is almost cylindrical.

The distal extremity of the humerus is provided with two articular condyles, one of which articulates with the ulna and the other with the radius. On the radial side of the palmar surface there is a ridge; and on the ulnar side of the same surface there is a second ridge diverging to the opposite tuberosity. The radial surface is a narrow, subelongate convexity, extending from the middle, approximately, of the palmar surface, obliquely to the lower part of the radial tuberosity. The two articular convexities, or trochlea, at the distal end of the humerus are bent toward the palmar aspect, the anconal aspect is the side on which the elbow is situated. The inner convexity is the larger and articulates with the ulna. To the outside is the processus cubitalis humeri. The outer articulates with the radius and is so arranged that the radius makes a greater flexion than the ulna. At the lower end of the humerus there is a depression which receives the anconeus of the ulna during flexion and extension of the forearm. On the shoulder- and elbow-joint are found grooves over which the tendons glide, at which places sheaths are provided. This arrangement also aids in keeping the capsular ligament in place.

=The Ulna.= Location.—The ulna, larger than the radius, is bent and articulates with the radius only at the ends. The two bones are bound together by a ligamentous band. It also articulates superiorly with the distal end of the humerus, and inferiorly with the carpus. When the wing is folded the radius is superior and a trifle to the inner side of the ulna.

Description.—The proximal end is most expanded, and is obliquely truncate for the articular excavation adapted to the ulnar tubercle of the humerus. A short angular process behind the cavity represents the olecranon, which is the processus olecranalis coracoideus ulnæ to which the ligamentum capsulare cubiti attaches. On the inner side of the head is the internal tubercle of the ulna and on the external side the external tubercle of the ulna. An excavation is noted on the radial side of the proximal end for the lateral articulation of the head of the radius. On the upper side near the articular part is located the sigmoid cavity.

The shaft, or corpus ulnæ, decreases in size near the distal end. It is slightly curved, flattened laterally with an internal and an external ridge.

The distal end of the ulna is slightly expanded into a trochlea which is sharply convex and articulates with two free carpal bones, the scapho-lunar, or os carpi radiale, and the cuneiform, or os carpi ulnare. The scapho-lunar is placed on the radial side, and the carpiulnare on the ulnar side. The distal extremity of the ulna is provided with a styloid process and on the dorsal side with a tubercle.

=The Radius.= Location.—The radius lies beside the ulna with which it articulates at each extremity. At the inferior extremity the articulation is rotary. It also articulates superiorly with the humerus and inferiorly with the os carpi radiale.

Description.—The radius, cylindrical in shape, is more slender than the ulna.

The proximal end is expanded, subelliptical, with a concavity for the oblique tubercle and a thickened convex border for articulation with the ulna. This end is provided with the tuberositas radii.