26 INTRODUCTION. 



The only method by which physiology can be studied is the observa- 

 tion of nature. The phenomena presented by living creatures are only 

 to be learned by direct examination, and cannot be inferred, by any 

 process of reasoning, from any other facts of a different character. 

 Even a knowledge of the minute structure of a part, however exact, 

 cannot furnish any information as to its active properties or function ; 

 and these properties can be learned only by examining the organ when 

 it is in a state of activity. Thus the muscular fibre and the nervous 

 fibre present certain well-defined characters of minute structure which 

 are easily distinguished by anatomical examination, but which could 

 not teach us anything of their physiological properties ; while direct 

 experiment shows that the muscular fibre is contractile and the nervous 

 fibre excitable or sensitive. 



Since the vital phenomena of the entire body result from the com- 

 bined activity of its different parts, thse different parts should be 

 studied by themselves in order to ascertain their particular properties. 

 This can be done by examination and experiment for each part while it 

 still retains its vital powers. Experience shows that after the circula- 

 tion has ceased, and consciousness and volition have disappeared, many 

 minute portions of the body continue for a time capable of manifesting 

 their physiological action. Thus a muscular fibre, separated from the 

 remaining tissues, may still be made to contract under the appropriate 

 stimulus; and a nerve, though cut off from its connection with the 

 brain, may also be called into activity by mechanical or electrical 

 irritation. This is because each part retains its physiological powers 

 so long as it retains its peculiar structure and constitution. The 

 general functions of the body, such as the circulation, digestion, and 

 respiration, have for their object to provide for the nutrition of the 

 tissues and organs, and thus maintain their natural constitution unim- 

 paired. Their cessation, accordingly, does not instantly destroy the 

 vitality of particular parts, but only after a sufficient time has elapsed 

 to alter or impair their natural constitution. The time during which 

 the vital powers may thus be retained varies* for different parts. Thus 

 the muscular fibre is capable of manifesting its excitability, as a general 

 rule, longer than the nervous fibre, and the nervous fibre longer than 

 the gray matter of a nervous ganglion. There is, also, a difference in 

 the same part shown by different kinds of animals. The excitability of 

 both nervous and muscular tissues continues longer in the cold-blooded 

 than in the warm-blooded animals, and in the quadrupeds longer than in 

 birds. In every instance, of course, the examination of such an isolated 

 part of the body should be made while it still preserves its physiological 

 properties. 



On the other hand, the functions of entire organs, or the general 

 functions of the body as a whole, can only be studied with success 

 while life is going on. The anatomical relations of the various organs 

 may be learned by dissection after death ; but their vital actions are 

 not to be ascertained in this way, because they have ceased and cannot 



