CERVICAL YEETEBEJ3. 



127 



for protecting the central trunks of the nervous and vascular 

 < ins, and to which, likewise, sometimes, appendages are 

 attached. A typical vertebra consists of a centre (centrum), 

 and ten processes (apophyses). From the upper part of 

 the centrum rise two neurapophyses, which form an arch 

 for enclosing the spinal cord and brain. These are sur- 

 mounted by a spine, called the neural spine. From the sides 

 of the centrum two transverse processes, or parapophyses, pro- 

 ject, which sometimes carry ribs, or pleurapophyses. From the 

 under side of the centrum two processes descend to enclose the 

 vascular trunks, in the same manner as the neurapophyses en- 

 close the spinal cord, they are called hcemapophyses ; from 

 them descends a single hcemal spine. The vertebral elements un- 

 dergo various phases of development in the different classes, 

 and in different regions of the spinal column of the same animal ; 

 it is therefore only by taking a philosophical view of their 

 structural development in the animal series that we obtain a 

 knowledge of the beautiful law which produces such endless 

 variety out of a few simple elements. 



[ 254. THE CERYICAL VERTEBRA (figs. 106 and 107) are 



Fig. 106. Fig. 107. 



others. We ob- 

 serve in them a 

 deviation from 

 the typical form 

 existing in the 

 dorsal region, 

 fig. 105 ; the 

 transverse pro- 

 cesses, fig. 



107 (g, g), parapophyses, and ribs, pleurapophyses, are rudi- 

 mentary, and soldered together, forming a hole (8), through 

 which the vertebral artery passes to the brain; the hcema- 

 pophyses are absent. This explanation of the structure of the 

 transverse processes of the cervical vertebrae is beautifully 

 illustrated in the neck of struthious birds. In all mammals we 

 find seven cervical vertebrae. The first vertebra of the neck, 

 the atlas (figs. 108 and 109), supports the skull ; it is more 

 moveable than the others, and differs considerably from the 

 typical form; the centrum (z) is much reduced to -receive a 

 toothlike process, rising from the centrum of the second ver- 

 tebra (fig. 1 1 0, k) ; around this pivot the atlas revolves, and 



