Part 43

INTERNEURAL ARTICULATIONS

Each typical vertebra presents two pairs of articular processes, which form diarthroses with the two adjacent vertebræ. The =articular surfaces= are extensive, flat, and oval in the cervical region, small and flat in the thoracic region, while in the lumbar region the anterior ones are concave and the posterior convex. The joint capsule is strong and ample in the cervical region, in conformity with the large size and greater mobility of these joints in the neck. In the thoracic and lumbar regions the capsule is small and close. These joints are arthrodia.

Associated with these joints are the =ligamenta flava=, which connect the arches of adjacent vertebræ. They are membranous and consist largely of elastic tissue.

FIG. 140.—LIGAMENTUM NUCHÆ OF HORSE.

1, Scapula; 1′, cartilage of scapula; 4, lamellar part of ligamentum nuchæ; x, wing of atlas. (After Ellenberger-Baum, Anat. für Künstler.) ]

The =supraspinous ligament= extends medially from the occipital bone to the sacrum. From the withers backward it consists of a strong cord of fibrous tissue, attached to the summits of the vertebral spines. In the neck and withers it is remarkably modified to form the ligamentum nuchæ, which requires more extended notice.

The =ligamentum nuchæ= is a powerful elastic apparatus, the principal function of which is to assist the extensor muscles of the head and neck. It extends from the occipital bone to the withers, where it is directly continuous with the supraspinous ligament. It consists of two parts—funicular and lamellar. The =funicular part= (Pars occipitalis) arises from the external occipital protuberance and the fossa below it, and is inserted into the summits of the vertebral spines at the withers. Two =bursæ= are usually found under it in the adult. The =supra-atloid bursa= lies between the ligament and the dorsal arch of the atlas. The =supraspinous bursa= occurs usually over the fourth thoracic spine, but may be over the third and may extend to the fifth. In the neck the funicular part consists for the greater part of two bands closely applied and attached to each other. Near and at the withers it broadens greatly, forming an expansion three to five inches (ca. 8 to 12.5 cm.) in width, the lateral margins of which are thin and overlie the trapezius and rhomboideus muscles. Behind the higher spines it becomes narrower and thinner, and is continued by the white fibrous lumbo-dorsal ligament. A mass of fat and elastic tissue lies upon the ligament as far back as the withers. It varies greatly in amount in different subjects, and is most developed in stallions of draft breeds, in which it forms the basis of the “crest.” The =lamellar portion= (Pars cervicalis) consists of two laminæ separated medially by a layer of loose connective tissue. Each lamina is formed of digitations which arise from the second and third thoracic spines and from the funicular portion, are directed downward and forward, and end on the spines of the cervical vertebræ, except the first and last. The digitation which is attached to the spine of the axis is very thick and strong. Behind this they diminish in size and strength; that to the sixth cervical is quite thin and feeble, or may be absent.

The =interspinous ligaments= (Ligamenta interspinalia) extend between the spines of contiguous vertebræ. In the cervical region they are narrow elastic bands, and in the thoracic and lumbar regions they consist of white fibers directed obliquely downward and forward.

The =intertransverse ligaments= (Ligamenta intertransversaria) are membranes which connect adjacent transverse processes in the lumbar region.

INTERTRANSVERSE ARTICULATIONS

These joints (peculiar to equidæ) are diarthroses formed by the transverse processes of the last two lumbar vertebræ and the alæ of the sacrum. A similar joint between the fourth and fifth lumbar processes is frequently present. The =articular surfaces= have an elongated oval form, the anterior one being concave and the posterior one convex. The capsule is tight, and is reinforced ventrally.

SACRAL AND COCCYGEAL ARTICULATIONS

In the foal the bodies of the five sacral vertebræ form joints which resemble somewhat those in the posterior part of the lumbar region. These joints are invaded by the process of ossification early, so that the consolidation of the sacrum is usually complete, or nearly so, at three years.

The coccygeal vertebræ are united by relatively thick intervertebral fibro-cartilages, which have the form of biconcave discs. Special ligaments are not present, but there is a continuous sheath of fibrous tissue. The movement in this region is extensive and varied. In old horses the first coccygeal vertebra is often fused with the sacrum.

MOVEMENTS OF THE VERTEBRAL COLUMN

The movements of the spine, exclusive of those at the atlanto-axial joint, are dorsal, ventral, and lateral flexion, and rotation. The range of movement at a single joint is very small, but the sum of the movements is considerable. The movements are freest in the cervical and coccygeal regions. Rotation is extremely limited in the thoracic and lumbar regions.

ATLANTO-AXIAL ARTICULATION

This is a trochoid or pivot joint, of a rather peculiar character. The articular surfaces are: (1) On the lateral masses of the atlas, two somewhat saddle-shaped facets, which are usually confluent ventrally; (2) on the axis, reciprocal saddle-shaped surfaces which extend upon the odontoid process and are confluent on its ventral aspect. It will be observed that the joint surfaces are not at all accurately adapted to each other, so that only limited areas are in contact at any time.

The =joint capsule= is attached around the margins of the articular surfaces. It is loose and ample enough laterally to allow extensive movement.

The =superior atlanto-axial ligament= (Ligamentum interarcuale) is membranous and reinforces the capsule dorsally.

The =interspinous ligament= (Ligamentum inter-spinale) consists of two elastic bands which extend from the dorsal arch of the atlas to the spine of the axis.

The =inferior atlanto-axial ligament= (Ligamentum dentis externum) arises from the ventral tubercle of the atlas and is attached by two branches on the ventral spine of the axis.

The =odontoid ligament= (Ligamentum dentis internum) is short, very strong, and somewhat fan-shaped. It extends from the rough concave dorsal surface of the dens or odontoid process, widens in front, and is attached to the transverse rough area on the inner surface of the ventral arch of the atlas.

=Movements.=—The atlas and the head rotate upon the axis; the axis of rotation passes through the center of the odontoid process and body of the axis.

FIG. 141.—ATLANTO-OCCIPITAL AND ATLANTO-AXIAL JOINTS OF HORSE, DORSAL VIEW AFTER REMOVAL OF DORSAL ARCH OF ATLAS.

a, Joint capsule of left part of atlanto-occipital joint; b, lateral ligament of same; c, c′, odontoid ligament; d, atlanto-occipital joint capsule; e, joint capsule of articulation between axis and third cervical vertebra; f, interspinous ligament; 1, occipital bone; 2, atlas; 3, axis; 4, third cervical vetrebra. (Ellenberger-Baum, Anat. d. Haustiere.) ]

THE ATLANTO-OCCIPITAL ARTICULATION

This joint may be classed as a ginglymus. The =articular surfaces= of this joint are: (1) On the atlas, two deep oval cavities; (2) the corresponding condyles of the occipital bone.