THE TISSUES 39 



of nerve and muscle the surroundings are able to act upon 

 the body, and the body can react upon its surroundings. 



These tissues may thus be called the Master Tissues, 

 and it is as the servants of these tissues that all the others 

 functionate. 



So far as the chemical changes in the body are concerned, 

 muscle is more important than nerve, for three reasons 

 First, it is far more bulky, making up something like 42 

 per cent, of the total weight of the body in man ; second, 

 it is constantly active, for even in sleep the muscles of 

 respiration, circulation, and digestion do not rest ; and third, 

 the changes going on in it are very extensive, since its great 

 function is to set free energy from the food. So far as the 

 metabolism of the body is concerned, muscle is the master 

 tissue. For muscle we take food and breath, and to get 

 rid of the waste of muscle the organs of excretion act. 

 Hence it is in connection with muscle that all the problems 

 of nutrition digestion, respiration, circulation, and excretion 

 -have to be studied. 



I. MUSCLE. 



The two great functions of muscle are 



To perform mechanical work. 



To liberate heat. 



The study of the physiology of muscle may be divided 

 into 



1. The structure, chemical and physical characters of 

 muscle at rest. 



2. The methods of making muscle contract. 



3. The changes which take place in muscle during con- 

 traction. 



4. The chemical change in muscle and the source of the 

 energy evolved. 



5. Death of muscle. 



1. STRUCTURE, CHEMICAL AND PHYSICAL CHARACTERS OF 

 MUSCLE AT REST. 



The first trace of the evolution of muscle is found among 

 the infusoria, where, in certain cells, in parts of the proto- 

 plasm, the network or cytomitoma is arranged in long parallel 



