THE RENAL ORGANS. 105 



tlic fixed and little distensible lungs, into the rerj dilatable 

 and compressible air-sacs. From these the act of expira- 

 tion expels it back through the principal bronchial pas- 

 sages to the trachea, and so out of the body. 



Both in Reptilia (e. g. Chelonia) and in Aves, muscular 

 filjres 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 

 llatitce, or struthiotis birds. So far as the contraction of 

 tb -se fibres tends to remove the ventral from the dorsal 

 walls of the lungs, they must assist inspiration. But this 

 diaphragmatic inspiration remains far weaker than the 

 sterno-costal inspiration. 



Finally, in the Mammalia, there are two equally important 

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

 phragmatic. The diaphragm, though it makes its appear- 

 ance in Sauropsicla, 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 

 towards the abdominal cavity, and convex towards the 

 thorax, the result of its contraction, and consequent flat- 

 tening, necessarily is to increase the capacity of the thorax, 

 and thus 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 sufiicient 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 

 independently of the other. 



The Renal Organs. — The higher Vertebrata are all provided 

 with two sets of renal organs, the one existing only during 

 the early fcetal state, the other persisting throughout life. 



The former are the Wolffian bodies, the latter the true 

 Kidneys. 



The Wolffian bodies make their appearance very early, on 

 each side of the ventral aspect of the spinal region of the 

 embryo, as small transversely-disposed tubuli, opening into 



