CHANGES IN A MUSCLE DURING CONTRACTION. 97 



be entering into contraction, and will be becoming negative toward the rest 

 of the fibre, including the part under A, whose negativity by this time is 

 passing off that is to say, B will now be negative toward A , and this will 

 be shown by a deflection of the needle in a direction opposite to that of the 

 deflection which has just previously taken place. Hence, between two elec- 

 trodes placed along a fibre a single wave of contraction will give rise to two 

 currents of different phases, to a diphasic change ; and this, indeed, is found 

 to be the case. 



This being so, it is obvious that the electrical result of tetanizing a 

 muscle when wave after wave follows along each fibre is a complex matter ; 

 but it is maintained that the apparent negative variation of tetanus can 

 be explained as the net result of a series of currents of action due to the 

 individual contractions, the second phase of the current in each contrac- 

 tion being less marked than the first phase. We cannot, however, enter 

 more fully here into a discussion of this difficult subject. 



Whichever view be taken of the nature of these phenomena, it is im- 

 portant to remember that the electrical changes are closely allied to the 

 chemical events involved in the contraction of the muscle and to the change 

 of form of the muscle. Their exact relations to each other await research. 



The Changes in a Nerve during the Passage of a Nervous Impulse. 



68. The change in the form of a muscle during its contraction is a 

 thing which can be seen and felt ; but the changes in a nerve during its 

 activity are invisible and impalpable. We stimulate one end of a nerve 

 going to a muscle, and we see this followed by a contraction of the muscle 

 attached to the other end ; or we stimulate a nerve still connected with the 

 central nervous system, and we see this followed by certain movements, or 

 by other tokens which show that disturbances have been set up in the cen- 

 tral nervous system. We know, therefore, that some changes or other, con- 

 stituting what we have called a nervous impulse, have been propagated 

 along the nerve ; but the changes are such as we cannot see. It is possible, 

 however, to learn something about them. 



69. The chemistry of a nerve. We have spoken of the medulla as fatty, 

 and yet it is in reality very largely composed of a substance which is not (in 

 the strict sense of the word) a fat. When we examine chemically a quantity 

 of nerve (or what is practically the same thing, a quantity of that part of the 

 central nervous system which is called white matter, and which as we shall 

 see is chiefly composed, like a nerve, of medullated nerves, and is to be pre- 

 ferred for chemical examination because it contains a relatively small quantity 

 of connective tissue), we find that a very large proportion, according to some 

 observers about half, of the dried matter consists of a peculiar body, 

 cholesterin. Now, cholesterin is not a fat but an alcohol ; like glycerin, how- 

 ever, which is also an alcohol, it forms compounds with fatty acids ; and 

 though we do not know definitely the chemical condition in which cholesterin 

 exists during life in the medulla, it is more than probable that it exists in 

 some combination w 7 ith some of the really fatty bodies also present in the 

 medulla, and not in a free isolated state. It is singular that besides being 

 present in such large quantities in nervous tissue, and to a small extent in 

 other tissues and in blood, cholesterin is a normal constituent of bile, and 

 forms the greater part of gall-stones when these are present ; in gall-stones it 

 is undoubtedly present in a free state. Besides cholesterin, " white" nervous 

 matter contains a less but still considerable quantity of a complex fat, whose 

 nature is disputed. According to some authorities rather less than half this 

 complex fat consists of the peculiar body lecithin, which we have already 

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