HUMAN PHYSIOLOGY. 



IXTEODUCTION. 



T)HYSIOLOGY is the study of the phenomena of life. It makes us 

 JL acquainted with their immediate causes, the conditions of their 

 manifestation, the material changes in the body by which they are 

 accompanied, their mechanism, and their results. It teaches us all 

 that can be known of the living organism in a state of activity, with 

 its different parts performing their appropriate functions, and the w T hole 

 structure exhibiting the characters of individuality and life. 



In physiology, as in all the other natural sciences, direct observation 

 is the only moans by which actual knowledge can be attained. Ample 

 experience has demonstrated that in these departments analogical 

 deductions and inferences are unsafe, and that every question must be 

 tested by experimental investigation. Even the anatomical structure 

 of an organ can never indicate with certainty its physiological proper- 

 ties, until by immediate examination we have found the function to be 

 associated with the structure. This method, which depends entirely 

 upon observation, is laborious and difficult ; but it is the method to 

 which we owe all our present knowledge of natural phenomena, 

 and the only one which can produce similar results in the future. 

 There are some special considerations regarding its application to 

 physiology, owing to the intricate constitution of organized beings, 

 and the complexity of their functions. 



The entire body is a composite structure, made up of many parts 

 with varied characters and properties ; and the life of the organism as 

 a whole depends on the combined activity of its different parts. Conse- 

 quently each one of these should be first studied by itself, in order to ascer- 

 tain, so far as possible, its individual characters. This may be done in 

 great measure by the examination of single parts, separated from the 

 rest ; because minute anatomical structures, like muscular fibres or nerve 

 fibres, owe their distinguishing properties directly to the nature and 



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