PART I. 



MUSCLE AND NERVE. CIRCULATION. RESPIRATION. 



ANIMAL HEAT. CENTRAL NERVOUS SYSTEM 



AND SPECIAL SENSES. 



THE PHYSIOLOGY OF MUSCLE AND NERVE. 



Introduction. Physiology, the study of the properties of living 

 organisms, can be properly appreciated and learned only when it is 

 approached from the practical and experimental side. The study of 

 the simplest forms of life, the unicellular organisms, is as yet only 

 in its infancy, and at the present moment experimental physiology 

 deals almost entirely with the functions of the various tissues and 

 organs which together make up a vertebrate animal. 



The cold-blooded vertebrate, such as the frog, is the most suitable 

 animal for elementary experiments ; it is cheap, readily obtained, and, 

 above all, its tissues under suitable conditions retain their vitality for 

 many hours after they have been excised and cut off' from their supply 

 of blood. Moreover, the law relating to experiments on animals 

 practically limits the experiments, which can be properly performed 

 by a medical student, to observations upon frogs. 



The muscular, nervous, and vascular systems of the frog are the 

 most important in an experimental course of physiology, for although 

 muscle and nerve are highly differentiated forms of protoplasm with 

 correspondingly characteristic functions, yet they show only in an 

 exaggerated way properties which are common to all living matter. 

 Thus in muscle the power of contraction or movement is highly 

 developed; in nerve the property of excitability or irritability, the 

 response to a stimulus. 



A 



