314 



PRACTICAL PHYSIOLOGY 



iliac end is divided into two portions (Fig. 191). Stimulation with a 

 weak induction shock at (a) or (a'}, when there are no nerve-fibres, will 

 produce a contraction of the one half of the muscle. Excitation, how- 

 ever, at (b) or (b'), where there are nerves, will evoke a contraction of 

 both halves. 



Gracilis Experiment. The gracilis muscle of the frog is in two por- 

 tions completely separated by a tendinous intersection (Figs. 21, 192). 

 Both halves of the muscle are supplied by a single nerve, the individual 

 fibres of which divide and supply both halves of the muscle. Stimula- 



Fio. 191. Diagram of the sartorius 

 experiment to show the transmission of 

 a nervous impulse in both directions. 



Fro. 192. Diagram of the gracilis 

 experiment to show the transmission of 

 a nervous impulse in both directions. 



tion of any kind at (a) or (a 1 ), where there are no nerve-fibres causes 

 only the corresponding half of the muscle to contract ; but excitation at 

 (b) or (&'), where the nerves lie, will cause both halves to contract. 



With this last experiment should be contrasted the so-called "para- 

 doxical contraction." 



Paradoxical Contraction. At the knee of the frog the sciatic nerve 

 divides into the peroneal and popliteal nerves, but the individual fibres 

 of the two nerves arise separately from the central nervous system and 

 lie alongside each other in the sciatic nerve (Fig. 193). As much of 

 the popliteal nerve (a) as possible is carefully dissected out ; the nerve 

 is divided and stimulated by a galvanic shock; the muscles supplied by 

 the peroneal nerve (b) will contract. This is due to the electrotonic 

 current, set up in nerve (a), stimulating the fibres of (b) where the two 

 nerves lie close together at (c). It is not due to the transmission of a 

 nervous impulse in both directions. 



Mechanical stimulation of the nerve (a) produces no polarisation and 

 therefore will not cause a contraction of the muscles supplied by the 



