Chapter IX 



PHYSIOLOGY OF TO-DAY 



by E. Kennerly Marshall, Jr. 



PHYSIOLOGY, in brief, is the study of function as dis- 

 tinguished from structure. When the physiologist 

 looks at a living organism, he asks, "What does it do 

 and how does it do it? What are its various parts for and 

 how do they perform their functions?" The knowledge 

 which has accumulated in regard to function is of two 

 kinds, descriptive and theoretical. The first consists in 

 statements of facts, as, for example, that the heart beats, 

 that stimulation of the vagus nerve causes a slowing of 

 the heart beat, or that muscular exercise causes an increase 

 in the breathing and heart rate. Such observations, if 

 accurately made, are incontrovertible, as they are mere 

 description. They may be simply qualitative or may 

 attempt to describe the phenomena in quantitative terms. 

 Much of the older physiology was necessarily of this type. 

 But after a certain number of facts have been accumulated, 

 it is essential for the progress of science that they be cor- 

 related and that explanations for the observed facts be 

 sought. The more recent physiology has, in large part, 

 been of this theoretical or explanatory nature. 



There are, in general, two modes of approach in attempt- 

 ing to explain the function of any particular organ, to 

 unravel the processes concerned in the functioning of that 

 part. One of these involves studying its variations in func- 

 tion in the intact normal animal, while the other involves 

 more or less interference with the normal animal by isolat- 



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