320 VERTEBRATE ZOOLOGY 



or toes. Each is composed of three phalanges. The first digit is 

 wanting, and the one opposite the calcaneum is the fifth. 



Exercise 42. Make a drawing of the bones of the hind leg, carefully 

 labeling all. 



The Axial Skeleton. The vertebral column consists of a suc- 

 cession of disklike vertebrae which are closely joined with one 

 another by intervertebral ligaments and thin, cartilaginous inter- 

 vertebral disks. The column can be divided into a number of 

 regions, there being seven cervical, twelve thoracic, seven lumbar, 

 four 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 lateroventrally 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 the articular processes by which the vertebrae are 

 firmly fitted together ; these are the zygapophyses. The anterior 

 processes, or prezygapophyses, project from the anterior end of a 

 vertebra and articulate with the posterior processes, or postzyga- 

 pophyses, of the vertebra in front of it. Between the neural proc- 

 esses 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 lacking. Note the two surfaces which articulate with 

 the occipital condyles of the skull ; note also the postzygapophyses. 

 A large foramen will be seen on each side, the foramen transver- 

 sarium, which forms the anterior end of the vertebrarterial canal. 

 The axis has a high and very much elongated spinous process, and 

 its centrum is continued anteriorly as a conical projection called 



