Part 6

The seventh by the long spinous process and the slender simple transverse process, and by the usual absence of the foramen transversarium.

LIGAMENTS OF THE VERTEBRAL COLUMN.

=Fibro-cartilagines intervertebrales.=--The separate vertebræ (except the atlas and axis) are united by the disk-shaped =intervertebral fibro-cartilages=, which are situated between the centra of the vertebræ. Each consists of a central pulpy portion and a fibrous outer portion, covered by strong intercrossing tendinous fibers which unite with the periosteum of the vertebræ.

=Ligamentum longitudinale anterius.=--On the ventral face of the centra of the vertebræ, from the atlas to the sacrum, lies a longitudinal ligament, the =anterior longitudinal ligament=. It is very small, almost rudimentary, in the cervical region: large and strong in the thoracic and lumbar regions.

=Ligamentum longitudinale posterius= (Fig. 14, a).--A corresponding ligament (=posterior longitudinal ligament=) lies on the dorsal surface of the centra (therefore within the vertebral canal). It is enlarged between each pair of vertebræ and closely united to the intervertebral fibro-cartilages.

=Ligamentum supraspinale.=--Between the tips of the spinous processes of the thoracic and lumbar vertebræ extend ligamentous fibers. They are not united to form a distinct band, and can hardly be distinguished from the numerous tendinous fibers of the supraspinous muscles. Together they represent the =supraspinous ligament=. From the tip of the spinous process of the first thoracic vertebra to the caudal end of the spine of the axis extends a slender strand representing the =ligamentum nuchæ= or =cervical supraspinous ligament=. It is imbedded in the superficial muscles of this region, some of which take origin from it.

Ligamentous fibers are also present between the spinous processes of the vertebræ (=ligamenta interspinalia=): between the transverse processes (=ligamenta intertransversaria=), and between the vertebral arches (=ligamenta flava=).

=Capsulæ articulares.=--The joints between the articular processes are furnished with =articular capsules= attached about the edges of the articular surfaces. These are larger and looser in the cervical region.

=Atlanto-occipital Articulation.=--The joint between the atlas and the occipital condyles has a single articular capsule, which is attached about the borders of the articular surfaces of the two bones. This capsule is of course widest laterally, forming indeed two partially separated sacs, which are, however, continuous by a narrow portion across the ventral middle line. This capsule communicates with that which covers the articular surface of the dens, and through this with the capsule between the atlas and axis. That portion of the capsule which covers the space between the ventral arch of the atlas and the occipital bone represents the =anterior atlanto-occipital membrane=; it is strengthened by a slender median ligamentous strand. The =posterior atlanto-occipital= membrane covers in the same way the space between the dorsal arch of the atlas and the dorsal edge of the foramen magnum. In it a number of different sets of fibers, with regard to direction and to degree of development, may be distinguished; these have sometimes been considered separate ligaments.

The =lateral ligaments= of the atlas begin at the lateral angle of the cranial margin of the atlas, at about the junction of its dorsal and ventral arches, and pass cranioventrad to the jugular processes.

=Articulation between the Axis and Atlas.=--The =articular capsule= is large and loose, being attached to dorsal and ventral borders of the atlas, about the articular surfaces of the axis, and to the cranial projection of the spine of the atlas. It also passes craniad along the ventral side of the dens and communicates here with the capsule of the atlanto-occipital articulation. In the dorsal part of the capsule a short strong ligamentous strand is developed, connecting the caudal border of the dorsal arch of the atlas with the tip of the cranial projection of the spinous process of the axis.

First three cervical vertebræ and base of the skull, with dorsal surface removed. a, ligamentum longitudinale posterius; b, transverse ligament of the atlas; c, ligamenta alaria; d, odontoid process; e, occipital condyles; 1, 2, 3, the first three cervical vertebræ; 4, basal portion of the occipital bone.]

The dens or odontoid process is held in place by the =transverse ligament= (Fig. 14, b) of the atlas, which passes across the process as it lies within the vertebral canal of the atlas. The transverse ligament is attached at its two ends to the medial surface of the sides of the atlas at about the region where the dorsal and ventral arches of the atlas unite.

From the cranial end of the odontoid process the two =ligamenta alaria= (Fig. 14, c) diverge craniolaterad to the rough ventromedial angle of the condyles of the occipital bone.

=II. RIBS. COSTÆ= (Figs. 1 and 15.)

The cat has thirteen pairs of ribs. One of the fifth pair (Fig. 15) may be taken as typical. It is a curved flattened rod of bone attached at its dorsal end to the vertebral column, and at its ventral end to a cartilage (=costal cartilage=, Fig. 15, f) which serves to unite it to the sternum.

The most convex portion of the bone is known as the =angle= (e). Each rib presents a convex lateral and a concave medial surface, a cranial and a caudal border. The borders are broad dorsad and narrow ventrad, while the surfaces are narrow dorsad and broad ventrad. The rib has thus the appearance of having been twisted.

The rib ends dorsad in a globular head or =capitulum= (a), by which it articulates with the costal demifacets of two contiguous thoracic vertebræ. Between the capitulum and angle on the lateral surface is an elevated area, the =tubercle=, marked by the smooth tubercular facet (c) for articulation with the transverse process of a vertebra. The constricted portion between the head and tubercle is known as the =neck= (=collum=) (d). The angle is marked by a projecting process (e) (=angular process=) on its lateral border, for attachment of a ligament.

a, head; b, tubercle; c, tubercular facet; d, neck; e, angle, with angular process; f, cartilage.]

The ribs increase in length to the ninth (the ninth and tenth are of the same length) and then decrease to the last. They decrease in breadth behind the fifth. The first is nearly in a dorsoventral plane, while the others have their dorsal ends inclined slightly craniad.

a, manubrium; b, the separate pieces forming the body; c, bony part of the xiphoid process (the expanded cartilaginous portion not being shown); d, facet for attachment of first rib.]

The tubercles become less prominent as we pass caudad and are absent on the last two or three ribs, which do not articulate with the transverse process.

The first nine ribs (=true ribs= or =costæ veræ=) are attached separately to the sternum by their costal cartilages. The last four (false ribs or costæ spuriæ) are not attached separately to the sternum. The costal cartilages of the tenth, eleventh, and twelfth are united to one another at their sternal ends. They may be united also to the ninth costal cartilage or to the sternum by a common cartilage of insertion, or they may be quite free from the sternum. The thirteenth costal cartilages are free (floating ribs).

=Ligaments of the Ribs.=--The articular surfaces between the head of the rib and the centra, and between the tubercle and the transverse process of the vertebra, have each an articular capsule. There are also a number of small ligamentous bands from the tuberosity and the neck of the rib to the transverse process of the vertebra.

=III. STERNUM.= (Fig. 16.)

The sternum consists of three portions, a cranial piece or =manubrium= (a), a caudal piece or =xiphoid= process (c), and a middle portion or =body= (=corpus=), which is divided into a number of segments (b).

To the sternum are united the ventral ends of the first nine ribs. It thus forms the median ventral boundary of the thorax. Since the thorax decreases in dorsoventral measurement craniad, the long axis of the sternum is inclined from its caudal end dorsocraniad, and if continued would strike the vertebral column in the region of the first cervical vertebra.

The manubrium (a) makes up about one-fifth the whole length of the sternum and projects craniad of the first rib. It has the form of a dagger and presents a dorsal surface and two lateral surfaces, the latter uniting ventrad to form a sharp angle. In the middle of the lateral surface near the dorsal margin is an oval articular surface (d) borne on a triangular projection. It looks caudodorsad and is for the first costal cartilage.