36 HAND-BOOK OF PHYSIOLOGY. 



Glycogen is diminished, and muscle sugar (inosite) appears; the extrac- 

 tives 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 op- 

 posite direction to those existing in the muscle at rest. This causes a, 

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

 site to the original current, and is called by some the negative variation of 

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



Conditions of Contraction. (a) The irritability of muscle is great- 

 est at a certain mean temperature; (b) after a number of contractions a 

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



FIG. 287. Muscle-curves from the gastrocnemius of a frog, illustrating effects of alterations in 



temperature. 



time disappears altogether when they are removed from the body or the 

 arteries are tied; (d) oxygen is used up in muscular contraction, but a 

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

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

 (Hermann). 



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

 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 their 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 irri- 

 tated, and of that only, ensues; and this contraction is instantaneous, and 

 ceases on the instant of withdrawing the irritation. But when any part 

 with unstriped muscular fibres, e.g., the intestines or bladder, is irritated, 

 the subsequent contraction ensues more slowly, extends beyond the part 

 irritated, and, with alternating relaxation, continues for some time after 

 the withdrawal of the irritation. The difference in the modes of con- 

 traction of the two kinds of muscular fibres may be particularly illus- 

 trated by the effects of the electro-magnetic stimulus. The rapidly suc- 

 ceeding shocks given by this means to the nerves of muscles excite in 

 all the transversely-striated muscles a fixed state of tetanic contraction as 

 previously described, which lasts as long as the stimulus is continued, and 

 on its withdrawal instantly ceases; but in the muscles with smooth fibres 



