500 MANUAL OF PHYSIOLOGY. 



In a compound nerve some response is obtained when either the 

 distal or proximal end is irritated. 



CHEMISTRY OF NERVE FIBRES. 



The axis cylinder of nerves is probably composed, as already 

 mentioned, of protoplasm ; further than that nothing is known 

 of its chemical properties. The medullary sheath yields certain 

 substances which are related to the fats, and can be extracted with 

 ether and chloroform. Among these is the peculiar compound 

 nitrogenous fat, lecithin containing phosphorus, also cholesterin, 

 cerebrin and kreatin. 



ELECTRIC PROPERTIES OF NERVES. 



Like muscle, nerves may be regarded as having a state of rest 

 and a state of activity, but the two states are not obvious in the 

 same striking way as they are in muscle, nor do we know much 

 of the physical properties of nerve. While at rest, however, it 

 shows electric phenomena similar to those which have already 

 been described as belonging to muscle tissue. These electrical 

 currents are contemporaneous with the life of the nerve, and 

 they undergo the same variation as occurs in muscle when the 

 nerve passes into the active state ; that is, when it transmits an 

 impulse. 



The so-called natural current of nerve is practically the same 

 as that of muscle, passing in the nerve to the central part from 

 the cut extremities of the fibre; that is to say, the current passes 

 through the galvanometer from the electrode applied to the 

 middle of the nerve to that applied to the extremity. The 

 electro-motive force of a small nerve is much less than that of a 

 muscle. In a frog's sciatic it has been estimated to be 0.02 of a 

 Daniell cell. The natural current of the frog's nerve is said to 

 increase in intensity in proportion to the increase in temperature 

 up to about 20 C., after which it decreases. 



Experiments on nerve currents must be carried on with all 

 the precautions mentioned in speaking of muscle currents, and 

 with the non-polarizable electrodes there figured (page 448). 



