04 THE ANATOMY OF VERTEBRATED ANIMALS. 



being shut, pumping it, by the elevation of the hyoidean ap- 

 paratus and floor of the pharynx, into the lungs. A Frog, there- 

 fore, cannot breathe properly if its mouth is kept wide open. 



In most ReiMlia, and in all Aves and Mammalia, the ster- 

 num and ribs are capable of moving in such a way as alter- 

 nately to increase and diminish the capacity of the thoracico- 

 abdominal cavity, and thereby to give rise to an inspiratory 

 and expiratory flow of air. 



In the Heptilia, the elastic lungs dilate with the inspira- 

 tory, and contract with the expiratory, act ; but, in Aves, the 

 air rushes through the principal bronchial passages of the fixed 

 and little distensible lungs, into the very dilatable and com- 

 pressible air-sacs. From these the act of expiration expels it 

 back through the principal bronchial passages to the trachea, 

 and so out of the body. 



Both in Heptilia (e. g., Ghelonia) and in Aves, muscular 

 fibres pass from the ribs to the surface of the lungs beneath 

 the pleuroperitoneal membrane, and this rudimentary dia- 

 phragm acquires a very considerable development in the Ma- 

 titm, or struthious birds. So far as the contraction of these 

 fibres tends to remove the ventral from the dorsal walls of the 

 lungs, they must assist inspiration. But this diaphragmatic in- 

 spiration remains far weaker than the sterno-costal inspiration. 



Finally, in the Mamm.alia, there are two equallj'-important 

 respiratory pumps, the one sterno-costal, the other diaphrag- 

 matic. The diaphragm, though it makes its appearance in 

 Sauropsida, only becomes a complete partition between the 

 thorax and the abdomen in mammals ; and, as its form is such 

 that, in a state of rest, it is concave toward the abdominal 

 cavity, and convex toward the thorax, the result of its con- 

 traction, and consequent flattening, necessarily is to increase 

 the capacity of the thorax, and thi3^ pump the air into the 

 elastic lungs, which occupy a large part of the thoracic cavity. 

 When the diaphragm ceases to contract, the elasticity of the 

 lungs is sufficient to expel the air taken in. 



Thus, mammals have two kinds of respiratory mechanism, 

 either of which is efficient by itself, and may be carried on' in- 

 dependently of the other. 



The Renal Organs. — The higher Vertebrata are all pro- 

 vided with two sets of renal organs, the one existing only dur- 

 ing the early foetal state, the other persisting throughout life. 



The former are the Wolffian bodies, the latter the true 

 Kidneys. 



