328 THE MECHANICAL FUNCTIONS. 



like form which we shall presently see exemplified in the 

 cetaceous tribes. The phalanges are also large and length- 

 ened, forming a kind of oval hand, or rather paddle, the 

 functions of which it is w^ell calculated to perform. The 

 curvature of the humerus is of great advantage to the .tor- 

 toise in assisting it to turn itself, when, by any accident, it 

 has been laid on its back. 



Considerable differences may be noticed in the structure 

 of the several species of Chelonia, according to the diversity 

 of their habits. Tortoises which live on land, require more 

 complete protection by means of their shell than turtles, or 

 Emydes, which dwell only in the water: hence the convex- 

 ity of their carapace, the solidity of its ossification, its im- 

 moveable connexion with the plastron, and the complete 

 shelter it affords to the head and limbs. Turtles, on the 

 other hand, receiving support from the element in which 

 they reside, require less provision to be made for these ob- 

 jects. Their carapace is smaller, has a more flattened form, 

 and cannot afford protection to the head and limbs. These 

 latter organs are proportionally larger, present a greater de- 

 velopment of the radius and ulna, and are compressed into 

 a flat expanded surface. Previously to the retraction of the 

 head and limbs within the shell, the air is expelled from the 

 large cavities of the lungs, by the vigorous actions of the ab- 

 dominal muscles, which exist in these animals as well as in 

 all the vertebrata, although here they are covered by the 

 bones, and compress the lungs by pushing the abdominal 

 viscera against them. This sudden expulsion of air is the 

 cause of the long continued hissing sound which the tortoise 

 emits while preparing to retreat into its strong hold. 



The ribs, though they first assume the form of broad 

 plates immoveably united to the spine, when they have pro- 

 ceeded a certain distance, separate from each other, and re- 

 sume their usual form; the intervening spaces between two 

 adjacent ribs being here filled up by membrane. The plas- 

 tron is united with the carapace by membrane, likewise; and 

 the sternum, instead of forming one broad plate of bone, has 



