THE CAT 263 



by intervertebral ligaments; thin cartilaginous disks also lie be- 

 tween them. The column may be divided into a number of regions, 

 there being seven cervical, thirteen thoracic, seven lumbar, three 

 sacral, and a variable number of caudal vertebrae. 



Each vertebra is a single bone, which has been formed by the 

 fusion of several distinct bones. The body of the vertebra, or 

 centrum, forms its ventral portion. The neural arch, which in- 

 closes the neural canal and is dorsal in position, is composed 

 originally of three bones,— the two lateral neural processes and 

 the median neural spine, or spinous process. Extending laterally 

 or ventrolateral^ from the centrum and neural process on each side 

 is the transverse process. Projecting from the anterior and also 

 from the posterior surface of the neural processes are articular 

 processes by which the vertebrae are firmly fitted together. These 

 are the zygapophyses. The anterior processes, or prezygapophyses, 

 project at the anterior end of a vertebra and articulate with the 

 posterior processes, or postzygapophyses, of the vertebra in front 

 of it. Between the neural processes of each two vertebrae on each 

 side is an opening called the intervertebral foramen, through which 

 the spinal nerve passes. 



The cervical vertebrae are not all alike. The first two are called 

 the atlas and axis, and support the skull. The atlas is a bony 

 arch, which consists of a ventral and two lateral portions, the 

 latter being the broad transverse processes. A centrum and a 

 spinous process are absent. Note the two surfaces which artic- 

 ulate with the occipital condyles of the skull; note also the 

 postzygapophyses. Two foramina will be seen on each side, the 

 foramen transversarium at its hinder end and the atlantal foramen 

 at its forward end ; note the groove which joins them. These struc- 

 tures form the anterior end of the vertebrarterial canal. The axis 

 has a high and much elongated spinous process, and its centrum 

 is continued anteriorly as a conical projection called the odontoid 

 process. This process projects into the atlas, whose centrum it 

 represents ; it is held in position here by the transverse ligament. 



The transverse processes of the cervical vertebrae are not 

 homologous to those of the other vertebrae but each is equiva- 

 lent to a transverse process plus the rudiment of a rib. Between 



