ON THE STUDY OF PHYSIOLOGY. 7 I 



sensibility and irritability, and the structures of living mus- 

 cular and nervous fibres. 



This language does not explain how the thing takes 

 place : it is merely a mode of stating the fact. To say that 

 irritability is a property of living muscular fibres, is merely 

 equivalent to the assertion, that such fibres have in all 

 cases possessed the power of contraction. What then. is 

 the cause of irritability ? I do not know, and cannot 

 conjecture. 



In physiology, as in the physical sciences, we quickly 

 reach the boundaries of knowledge whenever we attempt to 

 penetrate the first causes of the phenomena. The most 

 we can accomplish is, to make gradual conquests from the 

 territories of ignorance and doubt ; and to leave under their 

 dominion those objects only which our reason has not 

 reached, or is not able to reach. The great end of observa- 

 tion and experiment is to discover, among the various phe- 

 nomena, those which are the most general. When these are 

 well ascertained, they serve as principles, from which other 

 facts may be deduced. The Newtonian theory of gravita- 

 tion is a most splendid example. The only object of un- 

 certainty, which then remains, in the first cause of a small 

 number of facts. The phenomena succeed each other, 

 like the generations of men, in an order which we observe, 

 but of which we can neither determine nor conceive the 

 commencement. We follow the links of an endless chain-; 

 and, by holding fast to it, we may ascend from one link to 

 another ; but the point of suspension is not within the 

 reach of our feeble powers. 



To call life a property of organization would be unmean- 

 ing : — it would be nonsense. The primary or elementary 

 animal structures are endued with vital properties ; their 

 combinations compose the animal organs, in which, by 

 means of the vital properties of the component elementary 

 structures, the animal functions are carried on. The state 

 of the animal, in which the continuance of these processes 

 is evidenced by obvious external signs, is called life. 



The striking differences between living and inorganic 



