40 PSYCHOBIOLOGY 



Smooth muscle is supplied, like cardiac muscle, with a double set of 

 nerves, one heightening the excitability and the other depressing it. 



THE CHEMICAL PROCESS IN MUSCLE. 



The chief products of muscular activity are carbon dioxid, water, and 

 sarcolactic acid. Sarcolactic acid is isomeric with the lactic acid of sour 

 milk, but the former rotates the plane of polarization of polarized light to 

 the right, whereas the latter does not rotate the plane at all. It is prob- 

 able that the primary chemical activity which conditions muscular con- 

 traction is the breaking-down of some complex substances, which may be 

 highly unstable, although one physiologist supposes it to be grape sugar 

 (C 6 H 12 O ). The lactic acid (C 3 H 6 3 ) is then oxidized, the oxygen being 

 supplied by the red blood corpuscles. Oxidization is at any rate an im- 

 portant part of the chemical process in muscle, and through it is liberated 

 the heat, or at least a part of the heat, which is a noticeable consequence 

 of muscular activity. 



Normally, the greater part of the lactic acid produced is oxidized in the 

 muscle. If sufficient oxygen is not present the acid is thrown in abnormal 

 quantity into the circulation, and excreted by the kidneys. In normal 

 urine, from 3 to 4 milligrams of acid per hour are excreted. In one case, 

 the urine passed thirty minutes after running a third of a mile contained 

 454 mgs. of lactic acid. 



In the intervals of ' rest ' the muscle cell builds up the complex sub- 

 stance which is broken down in the period of ' activity '. According to 

 some physiologists the food material and oxygen are together built up into 

 an unstable compound, which is broken down in activity, forming C0 2 , 

 without additional oxygen. According to this theory, the account given 

 above is erroneous. 



FATIGUE. 



If a muscle be repeatedly stimulated, its contractions eventually become 

 less in extent, and the latent periods, as well as the periods of contraction 

 and relaxation — especially the latter — become prolonged. In the human 

 being this condition is accompanied by the experience of fatigue. 



Whatever may be the exact nature of the chemical processes in muscular 

 activity, it is probable that two factors contribute to fatigue: (1) the 

 partial exhaustion of the stored material which the cell has elaborated as 

 material for its contractile activity; (2) the accumulation of the products 

 of decomposition. These substances (lactic acid, carbon dioxid, etc.) have 

 an inhibitory or deadening effect on the muscle cell, and possibly some of 

 them affect the sensory nerve endings in the muscles, producing the experi- 



