PROPERTIES AND FUNCTIONS OF NERVES. 89 



possesses a delicate homogeneous membrane, the sarcolemma, in the inte- 

 rior of which is contained the contractile substance, which presents a 

 striated appearance. During life this substance is in a fluid condition, but 

 after death undergoes stiffening. 



The non-striated muscles form membranes which surround cavities, e.g., 

 stomach, arteries, bladder, etc. They are composed of elongated cells 

 without striations, and contain in their interior one or more nuclei. 



Muscular tissue is composed of water, an organic contractile substance, 

 myosin, non-nitrogenized substances, such as glycogen, inosite, fat, and 

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

 and after contraction it becomes acid. 



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 ^ 7 of a second, but varies with the temperature, the strength of 

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

 ever, are occasioned by contractions of a much longer duration, depending 

 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, 



