CHAPTER XXXI. 

 THE MUSCULAR SYSTEM. 



The General Properties of Muscular Tissues. The intimate 

 nature of the physical changes taking place during the contrac- 

 tion of a muscle are not understood, and the histological changes 

 which occur have had various interpretations put on them. For 

 a discussion of these a textbook on histology should Be consulted. 



The physiological property which distinguishes muscular tis- 

 sue from other forms of tissue is that of contractibility . It is to 

 this property that the forcible shortening of the muscles which 

 produces movements is due. The shortening occurs in the lonir 

 axis of the muscle and is accompanied by a compensatory thick- 

 ening in the transverse diameter, which keeps the bulk of the 

 muscle constant. After the period of active contraction the 

 muscle remains in the contracted position unless it be pulled back 

 into extension by some force. No isolated muscle can actively 

 expand ; it can only do so passively. Muscle does not possess the 

 property of initiating the contraction. This depends on the H-T 

 vous system acting on another property of muscle, namely, its 

 irritability, that is, the ability of the muscle to react very quickly 

 to a stimulus. The amount of stimulus which it requires is very 

 small compared with the reaction brought about in the muscl'e. 



A muscle can be stimulated in other ways than through its 

 nerve, namely, by mechanical, thermal, electrical, and chemical 

 stimuli applied directly to it. By using these artificial stimuli 

 on muscles excised from the body the properties of muscuhir 

 contraction can be studied. 



A record of the contraction of a muscle of a frog may be made 

 by excising it and attaching one end to a suitable clamp and the 

 other end to a light lever the opposite end of which is arranged 

 to trace on a smoked paper placed on a rapidly revolving drum. 

 If such a muscle be electrically excited, it will record its con- 

 traction as a curve on the smoked surface of the paper, and show 



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