16 THE PHENOMENA OF LIFE 



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 other 

 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 function; 

 from observations of the changes produced by experiment upon the various 

 processes in such animals, or in the organs and tissues of animals; from ob- 

 servations of the changes in the working of the human body produced by dis- 

 eases; 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 determination of 

 arterial blood pressure involves familiarity with extensive anatomical 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 ex- 

 tensive investigation at the present time require for their solution not only 

 the use of the most complex methods of chemistry, both analytical and synthet- 

 ical, but also the principles and methods of physics. Indeed, since the work 

 of Helmholz, the interpretation of physiological phenomena by means of physi- 

 cal laws and methods has contributed more than any other means toward the 

 prominent scientific position of physiology at the present time. In a word, 

 physiology must utilize the facts of anatomy, histology, biology, physics, and 

 chemistry to interpret the phenomena of life. 



