418 HANDBOOK OF PHYSIOLOGY. 



arising from some other source, (c) Certain imperfectly understood 

 chemical changes occur, in all probability connected with (a) and (#). 

 Glycogen is diminished, and glucose, or muscle sugar (inosite) appears; 

 the extractives are increased. 



(5.) Electrical changes. When a muscle contracts the natural 

 muscle current or currents of rest undergo a distinct diminution, which 

 is due to the appearance in the actively contracting muscle of currents in 

 an opposite direction to those existing in the muscle at rest. This causes 

 a temporary deflection of the needle of a galvanometer in a direction op- 

 posite to the original current, and is called by some the negative varia- 

 tion of the muscle current, and by others a current of action. 



Conditions of Contraction. (a) The irritability of muscle, as in^ 

 dicated by length of latent -period, velocity and extent of contraction, 

 is greatest at a certain mean temperature; (#) after a number of contrac- 

 tions a muscle gradually becomes exhausted; (c) the activity of muscles 

 after a time disappears altogether when they are removed from the body 

 or the arteries are tied; (d) oxygen is used up in muscular contraction, 



FIG. 295. Muscle-curves from the gastrocnemius of a frog, illustrating effects of alterations in 

 temperature. 



but a muscle will act for a time in vacuo or in a gas which contains no 

 oxygen: in this case it is of course using up the oxygen already in store; 

 (e) the contraction is greater if the stimulus is applied to the nerve, than 

 if it be applied to the muscle directly. 



Response to Stimuli. The two kinds of fibres, the striped and 

 the unstriped, have characteristic differences in the mode in which they 

 act on the application of the same stimulus; differences which may be 

 ascribed in great part to the respective differences of structure, but to 

 some degree, possibly, to their respective modes of connection with the 

 nervous system. When irritation is applied directly to a muscle with 

 striated fibres, or to the motor nerve supplying it, contraction of the 

 part irritated, and of that only, ensues; and this contraction is instanta- 

 neous, and ceases on the instant of withdrawing the irritation. But 

 when any part with unstriped muscular fibres, e. g. 9 the intestines or 

 bladder, is irritated, the subsequent contraction ensues more slowly, ex- 

 tends beyond the part irritated, and, with alternating relaxation, con- 

 tinues for some time after the withdrawal of the irritation. The differ- 

 ence in the modes of contraction of the two kinds of muscular fibres 

 may be particularly illustrated by the effects of the repeated stimuli 



