82 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



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 nerve-fibre, most rapidly of all. 



Protoplasmic Continuity is Essential to Conduction. Effect of a 

 Break in Protoplasmic Continuity. A break of protoplasmic continuity in any 

 part of a nerve- or muscle-fibre acts as a barrier to conduction. If a nerve be cut 

 through, the irritability and conductivity remain for a considerable time in the 

 severed extremities, but communication between them is lost, and remains absent 

 however well the cut extremities may be adjusted. The nerve-impulse is not 

 transmitted through the nerve-substance as electricity is transmitted along a 

 wire : join the cut ends of a wire, and the contact suffices for the passage of 

 the current ; join the cut ends of a nerve, and the nerve-impulse cannot pass. 

 Any severe injury to a nerve alters the protoplasmic structure and prevents 

 the chemical and physical processes through which conductivity is made 

 possible. It is probable that the same may be said of all forms of liv- 

 ing cells, and the absence of protoplasmic continuity would seem to be an 

 explanation of the fact that nerve- and muscle-fibres which lie close together 

 may physiologically act as separate mechanisms. 



Even in the case of apparently homogeneous protoplasm there is probably 

 a definite structural relation of the finest particles, and upon this the physi- 

 ological properties of the substance depends. Slight physical and chemical 

 alterations suffice to change the rate and strength of the conduction process, 

 and the power to conduct is altogether lost if the protoplasm is so altered that 

 it dies. 



The relation of conductivity to structure of cell-protoplasm is illustrated in 

 the effects of degeneration and regeneration upon the physiological properties of 

 the nerve-fibre. The life of the nerve-fibre is dependent on influences exerted 

 upon it by the nerve-cell of which it is a branch. When any part of the fibre is 

 injured it loses its power to conduct, and the portion of the fibre separated by 

 this block from its cell soon dies. The irritability and conductivity are wholly 

 lost at the end of three or four days, and the fibre begins to undergo degenera- 

 tion. The axis-cylinder and the myelin are seen to break up and are then 

 absorbed, apparently with the assistance of the nuclei which normally lie just 

 1 Biedermann : Elektrophysiologie, 1895, p. 137. 



