ELECTRO -Pit YS1OLOC Y 



729 



which indicate that physiological fac- 

 tors, as well as physical, are con- 



resistance of the physiological envelopes of such cells as the red blood- 

 corpuscles (p. 25), that the great transverse resistance of nerve, and 

 indeed the electrotonic currents, are due in part if not wholly to the 

 true resistance of one or more of its envelopes (perhaps the medullary 

 sheath) . Examples of such differences of resistance even in the fluid 

 constituents of one and the same animal structure are not wanting. 

 For instance, the specific resistance of the yolk of a hen's egg may be 

 three times greater than that of the white. 



The electrotonic currents cannot spread beyond a ligature ; they 

 are stopped by anything which destroys the structure of the tissue ; 

 they are affected by various reagents. But this does not prove that 

 they are other than physical in origin, for what destroys the structure 

 of the tissue or modifies its molecular condition may destroy or 

 diminish its capacity for polarization, or alter its electrical resistance. 



There are, however, certain facts 



physiological 

 physical, are < 



cerned. While the currents obtained 

 from core-models show a general re- 

 semblance to the electrotonic currents 

 of medullated nerve, there is one sig- 

 nificant difference : in the former the 

 katelectrotonic and anelectrotonic 

 currents are of equal intensity ; in 

 the latter the anelectrotonic prepon- 

 derates. The most probable explana- 

 tion is that the anelectrotonic current 

 of medullated nerve is made up of two 

 distinct electrical effects, one physio- 

 logical in nature, the other dependent 

 merely on the structure and physical 

 properties of the fibres, while the kat- 

 electrotonic current is wholly physi- 

 cal. It is in favour of this hypothe- 

 sis that under the influence of ether, 

 which abolishes the physiological 

 functions of nerve, the anelectrotonic 

 current diminishes till it becomes 

 equal to the katelectrotonic. Non- 



medullated nerves, in which the conditions for physical electrotonus, 

 if present at all, are only feebly developed, and which exhibit no kat- 

 electrotonic current, or only a very weak one, show an electrotonic 

 current, which is abolished by ether, and seems to represent the phy- 

 siological portion of the anelectrotonic current of medullated nerve. 



A nerve may be stimulated by an electrotonic current produced 

 in nerve-fibres lying in contact with it. A well-known illustration 

 of this is the experiment known as the paradoxical contraction 

 (Practical Exercises, p. 741). 



The current of action of a nerve can also stimulate another 

 nerve when the excitability of both is greater than normal, as 

 is the case in the nerves of frogs kept in the cold. This comes 

 under the head of secondary contraction. But the best-known 

 form of secondary contraction is where a nerve, placed on a 

 muscle so as to touch it in two points (Fig. 279), is stimulated 



FIG. 279. SECONDARY CONTRAC- 

 TION. 



The nerve of muscle M touches 

 muscle M' at a and b. Stimula- 

 tion of the nerve of M' at S causes 

 contraction of M. 



