GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 81 



It would seem as if there were a normal degree of irritability for each special 

 form of muscle-tissue, and as if either an increase or decrease of the irritability 

 above or below this level was a sign of deterioration. Exercise, if not excess- 

 ive, is favorable to the maintenance of this normal physiological condition. 

 Without doubt many of the differences which we attribute to the muscles of 

 different men are really due to differences in the central nerve-cells, the action 

 of muscles, rightly interpreted, being rather an expression of central nervous 

 activity than the result of peculiarities of the muscles themselves. To exercise 

 the muscles is to exercise the nerve-cells, and the effects of exercise upon these 

 nervous mechanisms is of as much importance as the effect upon the muscles. 

 In admiring visible proportions we must always bear in mind "the power 

 behind the throne." "Beef" is of use to the athlete, but the muscles are 

 merely the servants, and can accomplish nothing if the master is sick. The 

 nerve-cells always give out before the muscles, and the man preparing for a 

 contest should watch his nervous system more than his muscles. He who 

 forgets this can easily over-train, and do himself a permanent injury, besides 

 failing in the race. 



Effect of Enforced Rest. Not only is the strength of the muscles greatly 

 increased by exercise, but a lack of exercise soon results in a loss of strength. 

 This is seen when an individual is confined to his bed for even a comparatively 

 short time, or 'when a limb is subjected to enforced rest by being placed in a 

 splint. The cause is to be sought in changes peculiar to the muscle proto- 

 plasm itself, although reduced circulation may also play a part. The effect of 

 prolonged rest on the irritability of muscles, is seen most markedly when they 

 are separated from the central nervous system by injuries of their nerves (see 

 p. 79). The lowered irritability which results from prolonged rest is not 

 peculiar to muscles, but is shared by all forms of protoplasm. 



0. CONDUCTIVITY. 



Conductivity is that property of protoplasm by virtue of which a condition 

 of activity aroused in one portion of the substance by the action of a stimulus 

 of any kind may be transmitted to any other portion. For example, if the 

 edge of the bell of a vorticella (see Fig. 2, p. 34) be irritated by a hair, not 

 only do the movements of the cilia cease, but the contractile substance of the 

 bell draws it into a more compact shape, and the fibrillaB of the stalk shorten 

 and pull the bell away from the offending irritant. In such a case an exciting 

 process must have been transmitted throughout the cell, and through several more 

 or less differentiated forms of protoplasm. This property of conductivity is not 

 known to be limited to any one peculiar structural arrangement of protoplasm 

 distinguishable with the microscope, but is exhibited by a vast variety of forms 

 of cell-protoplasm, and by plants as well as animals. The cytoplasm of cells, 

 the part of the protoplasm surrounding the nucleus, appears to be composed 

 of a semifluid granular material, traversed in all directions by finest fibrillaB 

 which in some cases appear to form an irregular meshwork, the reticulum, and 

 in others to be arranged side by side as more or less complete fibrils. It is not 



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