78 ON THE STUDY OF PHYSIOLOGY. 



to our stomach, Is, in structure and action, the instrument 

 of mastication ; and, as birds have no teeth, it is the only 

 instrument for dividing the hard grain on which they feed. 

 Further inquiry shews, tliat even in tliis stomach, which is 

 covered by a thick insensible cuticle capable of bearing the 

 friction of grain and siliceous pebbles, digestion is really 

 eflFected, as in the stomach of man, by solution ; the solvent 

 juice being secreted by the large collection of glands at 

 the cardiac end of the oesophagus, and having an operation 

 similar to that of the gastric fluid of quadrupeds. 



It has been argued, that the arteries of the mammalia 

 must have a contractile power, because, in some worms 

 without a heart, these vessels carry on the circulation alone. 

 The whole economy is too different in the two instances to 

 admit of inferences from -analogy ; the circulating appara- 

 tus, in particular, is formed on plans altogether different in 

 the two cases ; and the structure and actions of the vessels 

 of worms are, in fact, very little known. 



Because the vesiculee seminales in some animals do not 

 communicate with the vasa deferentia, and therefore can- 

 not receive the fluid secreted in the testicles, it has been 

 inferred that they do not serve the purpose of reservoirs for 

 the seminal secretion in man ; where, however, they have 

 so free a communication with the vasa deferentia, that any 

 fluids pass into and even distend the former, before they go 

 on into the urethra. The organic arrangement is different 

 in the two instances ; and this difference leads us to expect 

 a modification in the function, instead of authorising us to 

 infer that the same office is executed in exactly the same 

 manner in both cases. If we met with animals in whom 

 the cystic duct opened into the small intestines separately 

 from the hepatic, shall we therefore infer that the human 

 gall-bladder is not a receptacle for the hepatic bile ? 



Again, animals may be compared to each other. Each 

 organ must be examined in all the gradations of living be- 

 ings ; its modifications compared and surveyed in relation 

 to the varieties of other parts, before a just notion of its 

 functions can be formed. This kind of examination of the 



