1 6 THE PHENOMENA OF LIFE 



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, 

 both analytic and synthetic, but also the principles and methods of physics. 

 Indeed, since the work of Helmholz, the interpretation of physiological phe- 

 nomena by means of physical methods and laws 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, his- 

 tology, biology, physics, and chemistry to interpret the phenomena of life. 



