1 6 THE PHENOMENA OF LIFE 



animal body in which the one is made the means to a knowledge of the other 

 as an end, and vice versa, according to the aims and purposes of the student. 



An equally important essential to the right comprehension of the changes 

 which take place in the living organism is a knowledge of the chemical com- 

 position of the body. Here, however, we can deal directly only with the 

 composition of the dead body, and it is well at once to admit that there may 

 be many chemical differences between living and non-living tissues; but as it 

 is impossible to ascertain the exact chemical composition of the living tissues, 

 the next best thing which can be done is to find out as much as possible about 

 the composition of the same tissues after they are dead. This is the assist- 

 ance which the science of Chemistry can afford to the physiologist. 



Having mastered the structure and composition of the body, we are 

 brought face to face with physiology proper, and have to investigate the vital 

 changes which go on in the tissues, the various actions taking place as long 

 as the organism is at work. The subject includes not only the observation 

 of the manifest processes which are continually taking place in the healthy 

 body, but the conditions under which these are brought about, the laws 

 which govern them and their effects. 



Sources and Utilization of Physiological Material. It may be well 

 to mention as a preliminary that the physiological information which we have 

 at our disposal has been derived from many sources, the chief of which are 

 as follows: From actual observation of the various phenomena occurring in 

 the human body from day to day, and from hour to hour, as, for example, the 

 estimation of the amount and composition of the ingesta and egesta, the res- 

 piration, the beat of the heart, and the like; from observations upon ether 

 animals, the bodies of which we are taught by comparative anatomy approxi- 

 mate the human body in structure and may be supposed to be similar in func- 

 tion; from observations of the changes produced by experiment upon the 

 various processes in such animals, or in the organs and tissues of animals; 

 from observations of the changes in the working of the human body produced 

 by diseases; from observations upon the gradual changes which take place in 

 the functions of organs when watched in the embryo from their earliest 

 beginnings to their completed development. 



The physiologist, in order to utilize the sources of material, must be 

 familiar with the gross structure of the animals or parts of animals which he 

 proposes to use in experimental procedure. So simple a matter as the deter- 

 mination of arterial blood pressure involves familiarity with extensive ana- 

 tomical structure. Experimental procedure must also draw on the field of 

 microscopic structure or histology, and many of the most instructive bodies of 

 physiological knowledge have come directly from the utilization of the facts 

 of comparative anatomy and of biology. The problems in animal nutrition 

 which are under such extensive investigation at the present time require for 

 their solution not only the use of the most complex methods of chemistry, 



