MUSCLE AND NERVE 183 



of a nerve fiber from its cell body, and by the effects of functional 

 activity. The metabolism of a cell depends upon the presence 

 of nuclear matter, so that if a nerve fiber is cut off from its 

 cell body it will degenerate. During this process there is 

 first an increase and then a decrease of irritability. A muscle 

 separated from the central nervous system by section of its 

 motor nerve will also degenerate. After a period of two weeks 

 or so it responds better to stimuli of long duration than to 

 those of short duration, and gradually becomes less and less 

 irritable to the end of the seventh or eighth month. Functional 

 activity leads first to an increase of irritability, but later in 

 this case also to a decrease. This is attributable to both the 

 consumption of its store of nutriment and to the accumulation 

 of waste products. Some physiologists have drawn a distinction 

 between the results of these two processes. Waste products if 

 formed faster than they can be gotten rid of give rise to fatigue, 

 while the consumption of stored or available nutriment gives 

 rise to exhaustion. That waste products are formed during 

 activity is shown by an experiment of Mosso. Having found the 

 injection of a definite amount of blood from a rested dog into 

 the veins of another to be without effect, he repeated the injec- 

 tion by using the blood of a dog completely tired out by a hard 

 day's work. The dog receiving the blood showed all the signs of 

 extreme fatigue. 



It is said that in order that protoplasm may conduct, it 

 must be continuous. A break in the physiological continuity 

 of protoplasm, as when a nerve is crushed at one point, com- 

 pletely bars the conduction process. The cut ends of the nerve 

 when placed in contact will not transmit a nerve impulse, which 

 distinguishes the latter from an electrical current, which passes 

 readily from one part to, the other. A conduction process hav- 

 ing reached the boundaries of the cell in which it has originated 

 may cause the generation of a similar process in a contiguous 

 protoplasmic mass. This, for instance, is shown by the rela- 

 tions of the end brush of one cell to the dendrites of another. 

 But fibers of muscles and nerves do not stimulate their neigh- 

 bors normally as they lie side by side, since their sheaths pre- 

 vent even contiguity. That the conduction process passes in 

 both directions along the length of the fiber may readily be 

 seen in muscle, where it is accompanied by a change of form. 



