CHAP, ii.] THE CONTRACTILE TISSUES. 123 



chiefly to the blood vessels ; and the function of non-medullated 

 fibres had better be considered in connection with nerves of which 

 they form a large part, such as certain nerves going to blood 

 vessels and to secreting organs. But it may be stated that though 

 they possess no medulla they are capable of propagating nervous 

 impulses in the same way as medullated nerves; and this fact 

 may be taken as indicating that the medulla cannot serve any 

 very important function as an electric insulator. 



71. 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 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 

 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 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 am- 

 monia compound of some considerable complexity ; it is therefore 

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

 and 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 

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



