192 MUSCLE AND NERVE 



rents of rest according as they are flowing in the same direction 

 or in an opposite direction. The strength of the electrotonic 

 current is dependent upon the strength of the polarizing current, 

 the length of the region between the electrodes, and the condi- 

 tion of the nerve. A dead nerve does not manifest electrotonic 

 currents, and they may be stopped by a ligature or by crushing 

 the nerve. 



Rigor Mortis. Whenever a muscle dies, it undergoes a change 

 manifesting itself in a loss of translucency, of extensibility, and 

 elasticity, by the development of a gradual contraction, of 

 increased heat production and acidity. This change is called 

 rigor mortis. It usually affects the body in regular order, the 

 jaw, neck, trunk, arms, and legs being influenced one after the 

 other. In general, the more active the protoplasm the sooner 

 does it pass into rigor. During life the central nervous system 

 is continually sending impulses to the muscles, keeping them 

 in a slight state of tension called muscle tonus. If the muscles 

 are severed from the central nervous system by curare, the 

 development of rigor is delayed. Cold delays and warmth 

 (38 to 40 C.) favors rigor. 



The chemical changes that take place during the onset of 

 rigor may be briefly summarized as follows: There. is a coagu- 

 lation of protein material of the muscle plasma. Possibly 

 under the influence of an enzyme, myosin and myogen are 

 converted to the insoluble forms myosin fibrin and myogen 

 fibrin. The increased acidity is due, certainly, to the develop- 

 ment of lactic acid and carbonic acid; perhaps also to other 

 acid bodies. A change in the glycogen content by rigor has 

 not been definitely established. 



Rigor caloris is caused by the precipitation of various proteins 

 of the muscle by heat. 



Chemistry of Muscle and Nerve. Muscle plasma, in addition 

 to water, contains a great number of substances which are 

 difficult to classify. They may be grouped, however, as follows: 

 Proteins, carbohydrates and fats, nitrogenous waste products, 

 lactic acid, phosphocarnic acid, pigments, ferments, and inor- 

 ganic salts. 



The most important substances isolated from the myelin 

 of nerve fibers are lecithin, cholesterin, and cerebrosides. 

 Lecithin (C 42 H 84 NP0 9 ) is a waxy, yellowish, hygroscopic 



