254 PHYSIOLOGY OF MUSCLES AND NERVES. 



able, not the nerve-trunks or the muscle-substance, but 

 some part intermediate between these two. The diffi- 

 culty is to prove that this part is exactly the final termi- 

 nation of the nerves. Assuming that these poisons 

 disable some part which lies between the nerve-trunk 

 and the muscle, but not the very end of the nerve, then, 

 though all the phenomena explained above are quite 

 intelligible, yet no answer has been gained to the ques- 

 tion of irritability, which we are discussing. 



Considering now the characters of the nerve, and of 

 its passage into the nerve-fibre, it is easy to understand 

 why the poison does not take effect on the nerve-trunks. 

 The nerve-fibres receive but few blood-vessels, so that 

 the poison in solution in the blood can only reach them 

 slowly, and in very small quantity. Moreover, the 

 fatty medullary-sheath probably forms a sort of protec- 

 tive envelope round the axis-cylinder. But where the 

 nerve enters the muscle-fibre it loses the medullary 

 sheath : and just at this same point a very complex net 

 of blood-vessels is present. Probably, therefore, it is 

 exactly the terminal nerve-plate (or the corresponding 

 nerve-branchings in the naked amphibia) which is most 

 exposed to the attack of the poison. So long, however, 

 as it is impossible to prove that this is really the actual 

 end of the nerve-fibre, a chance is left open to the op- 

 ponents of the theory of irritability. 



Great pains have been taken to settle this point 

 with certainty. If a muscle poisoned with curare is 

 compared with a similar but un poisoned muscle, it ap- 

 pears that the former is less excitable ; that is, that 

 stronger irritants are needed to cause it to pulsate. 

 The explanation of this may be that the muscle-sub- 

 stance is excitable, but not so much so as the intra- 



