C. THE EFFECTORS. 

 MUSCLE. 



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

 muscle is more important than nerve, for three reasons — 1st. 

 It is far more bulky, making up something like 45 per cent, 

 of the total weight of the body in men, and 38 per cent, in 

 women. 2nd. It is constantly active, for even in sleep the 

 muscles of respiration, circulation, and digestion do not rest ; 

 and 3rd. 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 ofet rid of the waste of muscle the orcrans 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. 



Muscle is the great liberator of energy in the body, and 

 the energy is used — 



1. To perform mechanical work. 



2. To heat the body. 



I. Development and Structure. 



Most free protoplasmic units have the power of contract- 

 ting and expanding. A muscle cell or fibre is specially 

 developed to contract and relax in one direction and to use 

 a large proportion of the energy liberated in work. 



The first trace of the evolution of muscle is found among 

 the infusoria, where, in certain species, in parts of the 

 protoplasm, long parallel fibrils run in the direction in which 

 the cell contracts and expands. Such a development has 

 been termed a myoid. 



20S 



