42 



CONSCIOUS NERVOUS OPERATIONS 



supports the lower limbs. It lias eight openings, which 

 communicate with the canal inclosing the spinal cord and 

 permit the passage of spinal nerves. 



At the lower end of the spinal column is the coccyx, 

 formed by the union of four very small vertebrae into one 

 bone. It is that part of the skeleton which in the lower 

 vertebrate animals forms the tail. 



41. The Vertebrae vary somewhat in form, but are all 

 constructed upon the same general plan (Fig. 30). There 

 is a stout bony cylinder, called the 

 body, or centrum. To this solid cen- 

 trum is attached an arch, called the 

 neural arch, which forms, with the 

 back of the centrum, an inclosed 

 space named the neural ring. The 

 successive neural rings form in the 

 spinal column a long tube in which 

 the spinal cord may safely lie. 



From the back of the neural arch 

 extends a long bony projection 

 called the spinous process, and the 

 successive processes, or spines (Fig. 

 29), extending down the backbone, 

 give to it the name of spinal column. 

 Six other processes project from 

 vertebra: one on each side called transverse pro- 

 cesses ; two called anterior articular processes, extending 

 forward ; and two called posterior articular processes, 

 extending backward, to meet the corresponding processes 

 of the neighboring vertebrae. These processes form, by 

 means of the intervening cushion of cartilage and con- 

 necting ligaments, a joint permitting a slight amount of 

 motion. 



2 



Fig. 30. A dorsal verte- 

 bra shown in two posi- 

 tions. 



1 centrum. 2 processes. 

 3 neural ring. 



each 



