44 



ELEMENTARY ANA TOMY. 



[LESS. 



suture with each other and with similarly expanded ribs, to 

 form the " shell " or carapace. These plates are so externally 

 situate as to be invested only by a horny form of skin. 



FIG. 57. DORSAL SURFACE OF A SHELL OF A FKESH-WATER TORTOISE 

 (Emys}. 



i 8, expanded neural spines ; r 1 r^, expanded ribs. (The dark lines indicate 

 the plates of the horny investment of the skeleton.) 



The transverse processes generally articulate with the 

 tubercles of the ribs, while the sides of the bodies bear 

 the heads of the ribs. This mode of articulation, however, 

 is not constant even in man's own class, as in the posterior 

 thoracic vertebrae of the ordinary Dolphins, where there are 

 no surfaces for the heads of ribs, the ribs are attached to the 

 ends of the transverse processes only ; while in the Sperm 

 Whale group (Physeteridcz] the ribs are attached exclusively 

 to parts which answer to the capitular surfaces of man. 



The notches for the spinal nerves, as has been said, are 

 not constant structures, but sometimes are replaced by 

 direct perforations of the neural laminae. 



The flatness of the surfaces of the vertebral bodies in man 

 is a condition constant in his class, but in Birds these 

 surfaces are concave in one direction and convex in another. 

 A ball and socket or a bi-concave articulation is to be 

 found in yet lower forms. 



That the only articular vertebral processes are the 

 zygapophyses, is a character man shares with most of his 



