CHAPTER V 

 THE CONTRACTILE TISSUES 



SECTION 1 

 THE STRUCTURE OF VOLUNTARY MUSCLE 



THE most striking features in the continual series of adaptations to 

 the environment, which make up the life of an individual, are the 

 movements carried out by contractions of the skeletal muscles. In 

 fact, all the mechanisms of nutrition can be regarded as directed 

 to the maintenance of the neuro-muscular apparatus, i.e. of the 

 mechanism for adapted movement. With the growth of the cerebral 

 hemispheres, which determines the rise in the scale of animal life, the 

 skeletal muscles become more and more the machinery of conscious 

 reaction. Even the highest of the adaptations possessed by man, 

 those involving the use of speech, are impossible without some kind 

 of movement. A man's relation to his fellows, and his value in the 

 community, are determined by these higher muscular adaptations. 

 It is not, therefore, surprising that the organs of the body which 

 present in the highest degree the reactivity characteristic of all living 

 things should have early attracted the attention of physiologists and 

 have been the object of numberless researches directed to determining 

 the ultimate nature of the processes generally described as vital. 



The movements of the muscles are carried out in response to 

 changes aroused in the central nervous system by events occurring 

 in the environment and acting on the surface of the body. Every 

 movement of an animal is thus in its most primitive form a reflex 

 action, and involves changes in a peripheral sense organ, in an afferent 

 nerve fibre, in the central nervous system, and in an efferent nerve 

 fibre, before the actual process of contraction occurs in the muscle 

 itself and gives rise to the resultant movement (Fig. 33). If we are to 

 determine the nature of the changes involved in this reflex action, we 

 must be able to study them as they progress along the different elements 

 which make up the reflex arc. This analysis is facilitated by the fact 

 that we are able to arouse a condition of activity in the different 

 parts of the arc, even when isolated from one another. Thus we can 

 excite any given reflex movement by stimulation of the periphery 



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