302 



PHYSIOLOGY OF MUSCLES AND N^ERVES. 



probable that in one nerve-fibre a resistance to transmission 

 exists, and in another an avalanche-like increase. All the 

 facts are more easily and simjily exjDlained by assuming that 

 there is a resistance to transmission in all nerves, allowance 

 being at the same time made for the difference in the ex- 

 citability of different points in the nerve. 



Moreover the curve of excitability in the case of the 

 sciatic nerve is not a simple ascending line from the muscle 

 to the dorsal marrow. This nerve is found, as is shown in 

 fig. 72, by the union of several roots ; it then, at various 



Fig. 72. The sciatic xerve and calf-muscle of a frog. 



points, gives off branches which enter the muscles of the 

 upper leg, and then separate into two branches, one of which 

 provides for the calf-muscle {gastrocnemius), the other for the 

 flexor muscle of the lower leg. If various points of this 

 nerve are irritated in the living animal, the nerve having 

 been merely exposed and isolated from the surrounding parts, 

 but not sepai-ated from the dorsal marrow, it is very evident 

 that the excitability at the upper points is generally greater 

 than at the lower ; but points are also found in the course 

 of the nerve at which a greater excitabilitv exists than at the 

 points above and below, as also, on the contrary, a less ex- 

 citability than at the adjacent ])oints. Such irregulaiities 

 are most abundantly exhibited at the points where nerve- 

 branches separate from the main trunk, especially when these 

 branches have been cut away. This is partly due to elec- 

 trotonic infiuences {cf. p. 125 et scq. ; p. 215 et seq., Note 13). 

 The nerve- fibres which are cut generate a current which 



