214 



THE ARTICULATIONS OF THE HORSE 



vaded by the process of ossification early, so that the consolidation of the sacrum 

 is usually complete, or nearly so, at three years. 



The coccygeal vertebrae are united by relatively thick intervertebral fibro-cartil- 

 ages, 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 exten- 

 sive 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 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-AXUL ARTICULATION 



This is a trochoid or pivot joint of a rather peculiar 

 character. The articular surf aces are : (1) On the lateral 

 masses of the atlas, two somewhat saddle-shaped facets, 

 which are separated by a wide notch above and a narrow 

 one below; (2) on the axis, reciprocal saddle-shaped sur- 

 faces which extend upon the dens 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 dorsal atlanto-axial ligament (Lig. interar- 

 cuale) is membranous and reinforces the capsule dor- 

 sally. 



The interspinous ligament (Lig. interspinale) con- 

 sists of two elastic bands which extend from the dorsal 

 arch of the atlas to the spine of the axis. 



The ventral atlanto-axial ligament (Lig. dentis ex- 

 ternum) arises from the ventral tubercle of the atlas and 

 is attached by two branches on the ventral spine of the 

 axis. 



The ligament of the dens or odontoid ligament (Lig. 

 dentis internum) is short, very strong, and somewhat fan- 

 shaped. It extends from the rough concave dorsal sur- 

 face of the dens, 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 body of the axis. 



Fig. 2.32. — Atl-\xto-occipital A?n) 

 Atlanto - AXi-AL Joints of 

 Horse; Dohsal View after 

 Removal of Dorsal Ahch of 

 Atl,^s. 



a, Joint capsule of left part of 

 atlanto-occipital joint; b, lateral 

 ligament of same; c, c', ligament of 

 the dens; d, atlanto-axial joint cap- 

 sule: e, joint capsule of articulation 

 between axis and third cervical 

 vertebra; /, interspinous ligament; 

 1, occipital bone; jS, atlas; 3, axis; 

 4, third cervical vertebra; 6, dorsal 

 longitudinal ligament. (Elienber- 

 ger-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. 



