498 MANUAL OF PHYSIOLOGY. 



result, and that of the proximal end produces signs of pain. In 

 a compound nerve some response is obtained when either the dis- 

 tal 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 com- 

 pound nitrogenous fat, lecithin containing phosphorus, also cho- 

 lesterin, 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 mid- 

 dle 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 445).| 



