516 HANDBOOK OF PHYSIOLOGY. 



or doubly refractive elements become less refractive and the singly re- 

 fractive more bo (iig. 329). 



(4.) Chemical Changes. (a) The reaction of the muscle which is 

 normally alkaline or neutral becomes decidedly acid, from the develop- 

 ment of sarcolactic acid, (b) The muscle gives out carbonic acid gas 

 and takes up oxygen, the amount of the C0 2 given out not appearing to 

 be entirely dependent upon the taken in, and so doubtless in part 

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

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

 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 

 opposite to the original current, and is called by some the negative vari- 

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



Fig. 329. The microscopic appearances during a muscular contraction in the individual 

 fibrillae, after Engelmann. 1. A passive muscle-fibre; c to d = doubly refractive discs, with 

 median disc a b in it; h and g are lateral discs; f and e are secondary discs, only slightly doubly 

 refractive; fig. on right same fibre in polarized light; bright part is doubly refracted, black ends 

 not so. 2. Transition stage; and 3. Stage of entire contraction: in eacn case the right-hand 

 figure represents the effect of polarized light. (Landois after Engelmann.) 



Conditions which Affect the Characters of the Contraction. 



In addition to the factors already considered which influence the irri- 

 tability of muscle as such, these and others may affect the characters of 

 its contraction and hence the curve produced. 



Effect of Load. TVithin certain limits a muscle contracts more pow- 

 erfully when acting against resistance that is, when it is loaded. Be- 

 yond this point of maximum contraction, however, increase of load di- 

 minishes the height and duration of contraction and increases the length 

 of the latent period. 



Effect of Fatigue. As already stated, exercise increases the strength 

 of muscles, so that the first effect of contraction is to increase the height 

 of the curve; but if the stimulation be kept up and the muscle be made 

 to contract frequently, both the height and form of the curve are altered. 

 The latent period is lengthened, the height of the curve is lessened, and 



