532 NUTRITION OF FARM ANIMALS 



vated from the cerebro-spinal system and are under the con- 

 trol of the will. The muscles of the internal organs are chiefly 

 non-striated muscles, the heart being the conspicuous exception, 

 and are to a very limited degree subject to the will, being in- 

 nervated from the sympathetic nervous system. In the study 

 of work production, therefore, we have to do chiefly with the 

 phenonena of striated voluntary muscles. 



The physiology of the muscle and of muscular contraction 

 is a very complex subject and wide differences of opinion exist 

 regarding many aspects of it. All that is attempted here is to 

 outline such general features as seem necessary for a proper 

 comprehension of its relations to nutrition. 



629. Contraction.* When a suitable stimulus, which in 

 the living animal is usually a nerve stimulus, is applied to a 

 muscle it contracts, that is, it tends to grow shorter and thicker. 

 This change is brought about by a shortening and thickening of 

 the individual fibers of which the muscle is built up. A single 

 stimulus, such, for example, as that caused by the making or 

 breaking of an electric circuit, gives rise to what is known as 

 a simple muscular contraction or twitch. If such a stimulus is 

 repeated with sufficient frequency it produces a series of simple 

 contractions which fuse together, resulting in a state of contrac- 

 tion which continues, subject to the effects of fatigue, as long 

 as the stimulus acts. This form of muscular contraction has 

 received the name of " tetanus." In the living animal the 

 ordinary contractions of the muscles, brought about by the 

 nervous system, even those that seem but momentary, are 

 essentially tetanic in their character. 



The term contraction as used in connection with the physi- 

 ology of muscle does not, however, necessarily imply an actual 

 shortening of the muscle. Contraction may either be isotonic 

 or isometric. When the muscle in contracting overcomes a 

 constant resistance, as, for example, in raising a weight, the 

 contraction is said to be isotonic. When, on the other hand, the 

 points of attachment of the muscle are fixed, evidently no work 

 can be done in the mechanical sense but the muscle still con- 

 tracts in the physiological sense, i.e., exerts a pull. Such a con- 

 traction is called an isometric contraction. 



630. Chemical changes in contraction. In a muscular con- 

 traction, either isotonic or isometric, there occurs a rapid 



