CHAP, ii.] THE CONTRACTILE TISSUES. 107 



is said to be faintly alkaline during life and to become acid after 

 death ; but in this grey matter nerve cells are relatively abundant ; 

 the white matter, composed chiefly of nerve fibres, is and remains, 

 during action as well as rest, and even after death, neutral or 

 slightly alkaline. 



Nor have we satisfactory evidence that the progress of a 

 nervous impulse is accompanied by any setting free of energy in 

 the form of heat. 



In fact, beyond the terminal results, such as a muscular con- 

 traction in the case of a nerve going to a muscle, or some affection 

 of the central nervous system in the case of a nerve still in con- 

 nection with its nervous centre, there is one event and one event 

 only which we are able to recognize as the objective token of a 

 nervous impulse, and that is an electric change. For a piece of 

 nerve removed from the body exhibits nearly the same electric 

 phenomena as a piece of muscle. It has an equator which is 

 electrically positive relatively to the two cut ends. In fact the dia- 

 gram Fig. 19, and the description which was given in 63 of the 

 electric changes in muscle may be applied almost as well to a 

 nerve, except that the currents are in all cases much more feeble 

 in the case of nerves than of muscles, and the special currents 

 from the circumference to the centre of the transverse sections 

 cannot well be shewn in a slender nerve ; indeed it is doubtful if 

 they exist at all. 



During the passage of a nervous impulse the ' natural nerve 

 current' undergoes a negative variation, just as the 'natural 

 muscle current' undergoes a negative variation during a con- 

 traction. There are moreover reasons in the case of the nerve, as 

 in the case of the muscle, which lead us to doubt the pre-exist- 

 ence of any such ' natural ' currents. A nerve in an absolutely 

 natural condition appears to be, like a muscle, isoelectric ; hence 

 we may say that in a nerve during the passage of a nervous 

 impulse, as in a muscle during a muscular contraction, a ' current 

 of action ' is developed. 



This ' current of action ' or ' negative variation' may be shewn 

 either by the galvanometer or by the rheoscopic frog. If the 

 nerve of the ' muscle nerve preparation ' B (see 64 ) be placed 

 in an appropriate manner on a thoroughly irritable nerve A (to 

 which of course no muscle need be attached), touching for 

 instance the equator and one end of the nerve, then single induc- 

 tion-shocks sent into the far end of A will cause single spasms 

 in the muscle of B, while tetanization of A, i. e. rapidly repeated 

 shocks sent into A, will cause tetanus of the muscle of B. 



That this current, whether it be regarded as an independent 

 ' current of action ' or as a negative variation of a ' pre-existing ' 

 current, is an essential feature of a nervous impulse is shewn by 

 the fact that the degree or intensity of the one varies with that 

 of the other. They both travel too at the same rate. In describ- 



