26 OSTEOLOGY. 



SPECIAL CERVICAL VERTEBRA. 



The Atlas. 



The First cervical vertebra is called tlie Atlas, because in 

 human anatomy it supports the head ; in quadrupeds the head 

 is suspended from it. It presents no well-defined body, but 

 consists of a strong ring of bone, the superior surface of -which is 

 convex, with a slight longitudinal elevation in the median line, 

 from which the alee or wings slope downwards and backwards. 

 The wings are large, flat transverse processes, wider than those 

 of any other true vertebra, and they terminate in rough convex 

 margins, which may be plainly felt through the skin in the living 

 animal ; they give insertion to portions of the levator humeri, 

 splenius, trachelo mastoideus, and obliquus capitis posticus muscles, 

 and are always largely developed in the carnivora. 



Superiorly, each ala exhibits three foramina ; the posterior is 

 the largest, and corresponds to the vertebral foramen, giving 

 passage to the anastomotic branch of the occipital artery. The 

 two anterior foramina are joined by a short, deep depression, 

 the alar gutter ; the external foramen gives passage to the 

 occipital artery, vein, and nerve, while the internal passes through 

 the ring into the neural canal, representing the intervertebral 

 foramen, and through it pass the cerebro-spinal artery, and the 

 first spinal nerve. 



The inferior surface of the atlas is convex centrally, and 

 presents posteriorly a short, obtuse eminence, which represents 

 the inferior spine, this is the tubercle of the atlas to -which the 

 tendon of the longus colli muscle is attached. On each side of 

 the central convexity we find a large concave space limited exter- 

 nally by the ala. The anterior surface presents two notches 

 and two articular depressions. The notches are of great size, and 

 contain the superior and inferior suspensory ligaments — one 

 being above, and the other below, the spinal canal. The arti- 

 cular glenoid cavities receive the condyles of the occipital bone, 

 and are each constructed of a superior and an inferior continuous 

 surface ; the inferior, r^resenting the convex head of other 

 vertebrae, is concave ; while the superior, nearly flat, represents 

 the oblique processes. A prominent bony ridge, to which is 

 inserted the investing <^t capsular ligament, surrounds the anterior 

 border of the bone. The posterior surface presents a somewhat 



