PROPERTIES AND FUNCTIONS OF NERVES. 99 



inorganic salts. When at rest the muscle is alkaline in reaction, but during 

 and after contraction it becomes acfd. 



Muscles possess the properties of (i) Contractility, which is the capa- 

 bility of shortening themselves in the direction of their long axis, and at 

 the same time becoming thicker and more rigid. (2) Extensibility, by 

 means of which they are lengthened in proportion to weights attached. 

 (3) Elasticity, in virtue of which they return to their .original shape when 

 the force applied is removed. 



The contractility of muscles is called forth mainly by nervous impulses, 

 descending motor nerves, which originate in the central nervous system ; 

 but it can also be excited by the electric current, the application of strong 

 acids, heat, or by mechanical means. 



Phenomena of a Muscular Contraction. When a single induction 

 shock is propagated through a nerve, the muscle to which it is distributed 

 undergoes a quick pulsation, and speedily returns to its former condition. 

 As is shown by the muscle curve, the contraction, which is at first slow, 

 increases in rapidity to its maximum, gradually relaxes and is again at rest, 

 the entire pulsation not occupying more than the ^ of a second. 



The muscular contraction does not instantly follow the induction shock, 

 even when the electrodes are placed directly upon the muscular fibres 

 themselves; an appreciable -period intervenes before the contraction, during 

 which certain chemical changes are taking place preparatory to the mani- 

 festation" of force. This is the " latent period," which has an average dura- 

 tion of the T ^ ff of a second, but varies with the temperature, the strength 

 of the stimulus, the animal, etc. The muscular movements of the body, 

 however, are occasioned by contractions of a much longer duration, depend- 

 ing upon the number (the average, 20) of nervous impulses passing to the 

 muscles in a second. 



During the muscular contraction the following phenomena are observed, 

 viz. : a change in form, a rise in temperature, a consumption of oxygen and 

 an evolution of carbonic acid ; the production of a distinct musical sound, 

 a change from an alkaline to an acid reaction, from the development of 

 sarcolactic acid; a disappearance of the natural muscle currents, which 

 under a negative "variation in the "latent period," just after the nervous 

 impulse reaches the termination of the nerve, and before the appearance 

 of the muscular contraction wave. 



Electrical Currents in Muscles and Nerves. If a muscle or nerve 

 be divided and non-polarizable electrodes be placed upon the natural 

 longitudinal surface at the equator, and upon the transverse section, electri- 



