40 PHYSIOLOGY FOR I!K< UNNKkS CH.M-. 



sacrum is formed in the infant as li\<- \eru-br.i , hut these in 

 adult life unite to form one hone. Since the arches of tilt- 

 vertebra: unite as well as the bodies and the processes, they 

 form by their union a canal running down the back of the 

 bone in which lie the nerves coming out from the end of the 

 spinal cord ; the cord itself ends at the level of the second 

 lumbar vertebra. 



The coccyx consists of a row of four small bones, the tippcr- 



FIG. 13. Articulation of a pair of ribs to a vertebra. 

 /', body of the vertebra ; //', transverse process ; r, rib ; c, costal cartilage ; sf, sternum. 



most one being united by a joint to the sacrum. In adult 

 life these four bones are usually united into one. They are 

 the rudiment of the many vertebra? of the tail in other 

 animals. 



The Bibs and Sternum. The ribs are twelve in number 

 on each side. Each rib is attached to the corresponding 

 dorsal vertebra, so that each dorsal vertebra carries a pair of 

 ribs. Each rib is articulated to the vertebra at two places, 

 at the transverse process of the vertebra, and on the body of the 



