CHAP, ii.] THE CONTRACTILE TISSUES. 105 



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 shew that disturbances have been set up in the central 

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

 constituting 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. 



66. The chemistry of a nerve. The medulla of a medullated 

 nerve fibre is usually spoken of as fatty, and yet 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 is chiefly composed, like a nerve, of medullated nerves, 

 and is to be preferred 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 the peculiar body cholesterin. Now 

 cholesterin is not a fat but an alcohol ; like glycerine, however, 

 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 with some of 

 the really fatty bodies also present in the medulla, and not in- a 

 fre^ isolated state. It is singular that besides being present in 

 su2h 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 seen to be present also in blood corpuscles and else- 

 where. Lecithin contains the radicle of stearic acid (or of oleic, 

 or of palmitic acid) associated not, as in ordinary fats, with simple 

 glycerine, but with the more complex glycerin-phosphoric acid, 

 and further combined with a nitrogenous body, neurin, an am- 

 monia compound of some considerable complexity ; it is therefore 

 of remarkable nature since, though a fat, it contains both nitrogen 

 arid phosphorus. 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 



