GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 77 



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

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



C. 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. 19) 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 fibrilhe 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 fibrillar 

 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 

 known whether the power of conduction is possessed by the whole of the pro- 

 toplasmic substance or is confined to the reticular substance, but there are cer- 

 tain reasons why the former view may be considered the more probable. The 

 rate and the strength of the conduction process varies greatly in different forms 

 of protoplasm, and there appear to be differences in the facility with which 

 the exciting process spreads through different parts of even the same cell. 1 Not 

 only are such differences to be noticed in many of the ciliated infusoria, but 

 even the substance of striated muscles seems to conduct in two different ways, 

 the sarcoplasm appearing to conduct slowly, and the more highly differentiated 

 fibrillary portion of the fibre rapidly. In general the process appears to be 

 more rapid and vigorous where a fibrillated structure is observable. Smooth 

 muscle-tissue, which has a somewhat simple structure, conducts comparatively 

 slowly; striated muscle, which is more highly differentiated, more rapidly, and 

 the fibrillated axis-cylinder of the ncrvc-libre, most rapidly of all. 



Protoplasmic Continuity is Essential to Conduction. — Effect of a 

 Break in Protoplasmic ( 'ontinmby.- — A break of protoplasmic continuity in any 

 1 Biedermann : Elektrophysiologic, 1895, 8. 137. 



