.350 THE MECHANICAL FUNCTIONS. 



In order to give an advantageous purchase to the 

 muscles which are attached to the spine, each ver- 

 tebra has, besides the parts above described, a pro- 

 jecting piece of bone, extending upwards from the 

 crown of the arch, and denominated the spinous 

 process (s, s). The sharp ridge that runs along the 

 middle of the back of a quadruped, is formed by 

 the continued series of these processes. There are 

 also, on the sides of the vertebrae, two other pro- 

 jecting pieces, which are denominated the transverse 

 processes ( r), and which serve as levers for bending 

 the column laterally, that is, either to the right or 

 to the left. All these component parts of the 

 spine are subject to considerable modifications, 

 in different tribes of animals, according to the par- 

 ticular mechanical circumstances of the system, 

 and to the particular intentions of their forma 

 tion. 



As it is in the spinal column more especially that 

 speculative naturalists have sought for illustrations 

 of their favourite doctrine of a strict unity of plan 

 in the conformation of the skeleton in all verte- 

 brated animals, it may be proper here to notice 

 the principles assumed by Geoffroy St. Hilaire as 

 regulating the formation of this part of the osseous 

 fabric* In common with all bones, the vertebrae 

 take their rise from certain determinate points, or 

 centres of ossitication, where, at first, detached 

 pieces of bone are formed, destined to unite toge- 

 ther so as to compose the entire bone. The num- 

 ber of elementary pieces which enter into the com- 

 position of a vertebra has been differently estimated 



* Memoires du Museum, ix. 79 and 89. 



