CHAPTER V. 

 THE CONTRACTILE TISSUES 



SECTION I 

 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 mechan- 

 isms 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 environ- 

 ment 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 contrac- 

 tion 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 excitr 

 any given reflex movement by stimulation of the periphery of the body, 

 or of the afferent nerve passing from the surface to the central nervous 



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