ON THE STUDY OF PHYSIOLOGY. ~ 77 



wliole mnclilnery In action. There are wliole classes of 

 living beings, and some of complicated structure, which have 

 no heart. 



Some have regarded the spleen as a spunge, soaking up 

 the blood when the stomach is ^mpty, and allowing it to 

 be squeezed out again by the pressure of this bag when 

 distended. In many animals, the spleen is neither cellular, 

 nor so situated as to be compressible by the stomach ; this 

 is the case, generally speaking, with birds and reptiles. The 

 office of conveying away fluids from the stomacli has been 

 assigned to it; making it a kind of water-pipe, to prevent 

 the liquid contents of the digestive cistern from rising above 

 a certain level. But it exists in reptiles and fishes, where 

 neither the figure of the stomach, nor the known habits of 

 the animals, in respect to food and digestion, admit of this 

 explanation. In the camel, which retains the water In Its 

 stomach, and in the horse, where It passes very rapidly into 

 the coecum, the spleen Is as large as in other animals. In 

 beasts of prey, which hardly drink at all, it is as large and 

 cellular as in the herbivorous ruminant animals. Its size 

 and Its cells are particularly conspicuous in the latter : yet 

 the fluids which they swallow go Into the paunch, and not 

 into the true digestive stomacli. 



Although arguments from analogy are of great service 

 in physiology and other departments of natural history, 

 although they throw light on obscure points, and give an 

 interest to many discussions, their employment requires 

 caution, and they should rather be resorted to for illustra- 

 tion than relied on for direct proof. Organs correspond- 

 ing In situation and name are not always constructed alike ; 

 hence a part is sometimes employed in one class of animals 

 for a different purpose from that which the instrument of 

 the same name and of analogous position in the body exe- 

 cutes In another. The gizzards of the gallinfe have a pro- 

 digious triturating power ; and those, who first ascertained 

 by experiment the extent of their power, were disposed to 

 infer that digestion is effected in man by mechanical attri- 

 tion. Now, the gizzard, although the corresponding part 



