THE DENTATA. 27 



triangular outline ; its inferior margin is excavated to receive the 

 odontoid process of the second vertebra, while the superior is 

 thin, and roughened for ligamentous insertion, and, on each side, 

 a broad slightly convex articular surface represents the posterior 

 oblique processes. This surface is bounded externally by a tidge, 

 which gives insertion to the capsular ligament ; and internally it 

 is separated by a similar ridge from the spinal canal. 



The spinal foramen of the atlas is very large, in order that the 

 spinal cord may be preserved from injury during the extensive 

 ,motions which take place between the head and neck. Its floor 

 is divided by a transverse ridge into two portions, the anterior of 

 which is roughened for the attachment of ligaments, the posterior 

 being smooth for the articulation of the odontoid process. Its 

 roof is smooth, and penetrated by two foramina for the passage of 

 an artery and nerves, and its sides have each an excavation in 

 which a venous sinus is lodged ; a foramen from this excavation 

 opens on the inferior surface of the ala, and transmits a vein from 

 the sinus. 



The atlas has three articular processes, — two for the occipital 

 condyles, and one for the axis, — all of which form true joints, 

 the atlas being the only vertebra possessing none but true 

 articulations. 



The Dentata. 



The Second vertebra is called also the axis, because the head 

 rotates upon it, also the vertebra dentata, from a process which, 

 in the corresponding human bone, somewhat resembles a tooth ire 

 shape, and is called the odontoid process ; it is situated on the 

 anterior end of the centrum, projecting forwards into the ring of 

 the atlas, and around it the head and atlas rotate. In the horse 

 the under surface of this process is smooth and rounded, and the 

 upper roughened and excavated, for the attachment of the 

 odontoid ligaments ; laterally, it terminates in a broad, oval, 

 somewhat convex surface, which represents the oblique processes, 

 and articulates with the posterior surface of the atlas ; these 

 articular shoulders being bounded by a ridge, to which the 

 capsular ligament is attached. 



The neural spine of the axis springs from a very strong arch, 

 and is nearly as long as the body, which is longer than that of 

 any other true vertebra ; this spine is convex, and consists 

 of lateral halves, united anteriorly into a strong rough crest, 



