Part 6

B. 1, Hemal or inferior spine (hypapophysis). 2, The neural arch. 3, The pleurapophyses or rib. 4, The superior or neural spine or neurapophysis which aids in the formation of the anapophyseal ridge. 5, Parapophyseal surface for head of rib. 6, Tubercle of rib articulating with the diapophysis. 7, Lamina of neurapophysis. 8, Centrum. 9, Hemal arch. ]

Between the bodies of the vertebræ are pads of fibrous cartilage. Above these bodies and inferior to the neural spines extends throughout the spinal column the large neural canal which is occupied by the spinal cord. Between the vertebral segments the neural canal is exposed superiorly, since here the neural arches do not completely bridge the canal. These spaces are protected by intervertebral ligaments.

In addition to the superior neural spines and the inferior spines, from the body of the vertebra project lateral processes, and anterior and posterior oblique processes.

All cervical vertebræ except the atlas and the axis are made up of the following parts: a body, or centrum, a neural canal, a neural arch, a superior dorsal, or neural spine, or neurapophysis, which in most cervical vertebræ is only slightly developed, two oblique processes, or zygapophyses, two transverse processes, or diapophyses (Fig. 10, No. 12) and in some vertebræ two tubercles, or anapophyses above the posterior zygapophyses. The prezygapophyses are directed upward and inward; the postzygapophyses are directed downward and outward. The inferior spines are only well developed in the last two or three cervical vertebræ.

Between vertebral segments, except the central sacral portion, on each side, there is an intervertebral foramen (Fig. 11, No. A, 4), through which pass the spinal nerves. In the central portion of the sacrum where the vertebræ have fused, there are two foramina on each side for each original vertebra, one located above the other, the upper giving passage to the sensory branch, the lower to the motor branch of the nerve.

=The Dorsal Vertebræ= (Fig. 4, No. 23; Fig. 11, A). Location.—The dorsal, or thoracic vertebræ, or vertebræ thoracicales, aid in forming the roof of the chest cavity.

Description.—The dorsal vertebral section is made up of seven vertebræ, with strong short bodies. The first and sixth dorsal segments articulate as do the cervical, by the bodies and oblique processes. (Fig. 11, No. A, 3, illustrates the oblique processes.) The seventh dorsal is fused with the first lumbo-sacral vertebra. The second to the fifth inclusive of the vertebræ are fused together, and the superior and inferior spinous processes are fused into a prominent plate-like ridge.

The transverse processes of the dorsal vertebræ, from the second to the sixth, are well developed and are bridged over with a thin layer of bone. The ventral spines are partly fused and form a very prominent and continuous ridge. (Fig. 11, No. A, 1, shows the fused superior spines; No. 2, the fused inferior spines; and No. 4, the intervertebral foramina.)

=The Lumbo-sacral Vertebræ= (Fig. 5, Part II, No. 4). Location.—The fused lumbo-sacral section of the spinal column forms the roof of the pelvic cavity.

FIG. 11.—Bones from the scapular arch.

A. Dorsal vertebra. 1, Superior spinous ridge, 2, Inferior spinous ridge. 3, Oblique processes. 4, Intervertebral foramina. 5, Articular facette for head of the rib. 6, Articular facette for tubercle of the rib. 7, Articular portion of the body.

B. Outer surface of scapula.

C. Inner surface of scapula. 1, Thin caudal end. 2, Articular head. 4, Processus furcularis. 5, Processus humeralis. 6, Processus coracoideus.

D. Outer surface of coracoid.

E. Inner surface of coracoid. 1, The furcular tuberosity. 2, Articular facette for articulation with the sternum. 3, Articular surface for humerus and scapula.

F. Outer surface of humerus and G the inner surface of the same. 1, Trochanteric fossa. 2, Oval articular head at proximal end. 3, Shaft. 4, Distal end showing articular condyles. 5, Trochanter. 6, Trochlea for ulna. 7, Trochlea for radius. 8, Depression or fossa.

H. Section through the median plane of the humerus showing the delicate cross partitions illustrating provisions made for bones provided with air-sac extensions. ]

Description.—The lumbo-sacral region consists of fourteen vertebræ which are distinct in the body of the newly hatched chick, but which become fused soon after hatching. With these vertebral segments there are fused the last dorsal and first coccygeal vertebral segments. This fusion is so complete that the segments are indicated only by the intervertebral foramina on the sides through which the spinal nerves pass, and by transverse markings on the inferior surface of the bodies.

The lumbo-sacral vertebræ, called the sacrum, and the ilia are fused. The dorsal spines of the vertebral segments are indicated only in the anterior portion where they are fused, forming a plate. There are no prominent ventral spines.

=The Coccygeal Vertebræ= (Fig. 4, No. 27). Location.—The coccygeal or caudal vertebræ, or vertebræ coccygeæ, constitute the bones of the tail.

Description.—There are seven coccygeal vertebræ. The last segment, shaped like a plough share and therefore called the pygostyle, is the largest and is supposed to have been formed by the fusion of several original distinct segments. It supports the coccygeal oil gland and the row of rudder feathers, or rectrices, which are arranged fan-wise.

All of the coccygeal vertebræ except the first are freely movable, thus allowing the tail to be used as a rudder during flight. The lateral spines are long and well developed, and the superior spines are bifurcated, thus giving increased surface for muscular attachment. The first coccygeal segment is fused with the last lumbo-sacral vertebra.

THE THORAX

The dorsal vertebræ superiorly, the ribs laterally, and the sternum, or breast-bone inferiorly, form the skeleton of a large cavity called the thorax. The dorsal vertebræ have been described.

=The Ribs= (Fig. 4, No. 32; Fig. 10, A and B). Location.—The ribs form the lateral bony wall of the thorax, articulating superiorly with the dorsal vertebræ.

Description.—The ribs are arranged in order of length, the ultimate rib being the longest. From anterior to posterior, they approach more nearly a horizontal position. The ribs are divided into the true and the false. The true ribs articulate with the sternum. The false ribs do not touch the sternum; they are floating. The true ribs are composed of two parts, a vertebral, or dorsal, and a sternal, or ventral. The vertebral part, or extremitas vertebralis, is provided with an articular head, or caput costæ; a neck, or collum costæ; and an articular tubercle, or tuberculum costæ. The head and tubercle articulate with the dorsal vertebra. Below the head of each rib is a pneumatic foramen.

The distal extremity of the dorsal section of the rib articulates with the proximal end of the sternal section by a diarthrodial articulation.

The articulations of the true ribs with the sternum is diarthrodial and each articulate by two small ridges with a double sternal facet. The first, the second, and the seventh ribs are floating, or false ribs. The first rib articulates with the quadrate part of the last cervical vertebra and also with the first dorsal vertebra. The seventh, the last rib, articulates with the under side of the anterior alar part of the ilium. This rib is situated similarly to the true ribs; but, instead of articulating directly with the sternum, the lower end lies against the sternal segment of the rib just anterior to it.