CHAP, ii.] THE CONTRACTILE TISSUES. 125 



In the ash of nerves there is a preponderance of potassium 

 salts and phosphates but not so marked as in the case of muscle 



72. The nervous impulse. The chemical analogy between 

 the substance of the muscle and that of the axis-cylinder would 

 naturally lead us to suppose that the progress of a nervous impulse 

 along a nerve fibre was accompanied by chemical changes similar 

 to those taking place in a muscle fibre. Whatever changes how- 

 ever do or may take place are too slight to be recognized by the 

 means at our disposal. We have no satisfactory evidence that in 

 a nerve even repeated nervous impulses can give rise to an acid 

 reaction or that the death of a nerve fibre leads to such a reaction. 

 The grey matter of the central nervous system it is true 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 connec- 

 tion 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 pheno- 

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

 positive relatively to the two cut ends. In fact the diagram 

 Fig. 20, and the description which was given in 66 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-existence 

 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 67) be placed in an 



