98 THE CONTRACTILE TISSUES. 



seen to be present also in blood corpuscles and in muscle. Lecithin contains 

 the radicle of stearic acid (or of oleic, or of palmitic acid) associated not, as 

 in ordinary fats, with simple glycerin, but with the more complex glycerin- 

 phosphoric acid, and further combined with a nitrogenous body, neurin, 

 an ammonia compound of some considerable complexity ; it is therefore of 

 remarkable nature, since, though a fat, it contains both nitrogen and phos- 

 phorus. According to the same authorities the remainder of the complex 

 fat consists of another fatty body, also apparently containing nitrogen but 

 no phosphorus, called cerebrin. Other authorities regard both these bodies, 

 lecithin and cerebrin, as products of decomposition of a still more complex 

 fat, called protagon. Obviously the fat of the white matter of the central 

 nervous system and of spinal nerves (of which fat by far the greater part 

 must exist in the medulla, and form nearly the whole of the medulla) is a 

 very complex body indeed, especially so if the cholesterin exists in combina- 

 tion with the lecithin, or cerebrin (or protagon). Being so complex it is 

 naturally very unstable, and indeed, in its stability resembles proteid matter. 

 Hence, probably, the reason why the medulla changes so rapidly and so pro- 

 foundly after the death of the nerve. 



The presence in such large quantity of this complex fatty medulla renders 

 the chemical examination of the other constituents of a nerve very difficult, 

 and our knovyledge of the chemical nature of, and of the chemical changes 

 going on in the axis-cylinder, is very limited. Examined under the micro- 

 scope the axis-cylinder gives the xanthoproteic reaction and other indications 

 that it is proteid in nature ; beyond this we are largely confined to inferences. 



After the fats of the medulla (and the much smaller quantity of fat 

 present in the axis-cylinder), the proteids of the axis-cylinder, and the other 

 soluble substances present in one or the other, or gathered round the nuclei 

 of the neurilemma, have by various means been dissolved out of a nerve 

 fibre, certain substances still remain. One of these in small quantity is the 

 nuclein of the nuclei ; another in larger quantity is the substance neuro- 

 keratin which forms, as we have seen, a supporting framework for the 

 medulla, and whose most marked characteristic is, perhaps, its resistance 

 to solution. 



In the ash of nerves there is a preponderance of potassium salts and 

 phosphates, but not so marked as in the case of muscle. 



70. The nervous impulse. The chemical analogy between the substance 

 of the muscle and that of the axis-cylinder would naturally lead us to sup- 

 pose that the progress of a nervous impulse along a nerve fibre was accom- 

 panied by chemical changes similar to those taking place in a muscle fibre. 

 Whatever changes, however, 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 reaction. The gray 

 matter of the central nervous system, it is true, is said to be slightly acid 

 during life and to become more acid after death ; but in this gray matter, 

 nerve cells are relatively abundant ; the white matter, composed chiefly of 

 nerve fibre, 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 contraction 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 connection 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 



