CHAPTER XVII. 



THE NERVE-TISSUE. 



OWING to the difficulty which attends the separation of the vari- 

 ous morphological components of nerve-tissue, and the peculiar prop- 

 erties of some of its most important chemical constituents, it is as 

 yet impossible to give an account of its chemical composition which 

 is at all satisfactory. Here, as in the other organs of the body, we 

 meet with certain substances which are generally spoken of as ex- 

 tractives, and which are manifestly katabolic products that result 

 during the functional activity of the tissue in question. These are 

 in no sense specific of nerve-tissue, however. They comprise kreatin, 

 uric acid, urea, xanthin, hypoxanthin, guanin, adenin, inosit, and 

 lactic acid viz., substances which, as we have just seen, are also 

 found in muscle-tissue. In addition, we find certain albumins 

 which in part belong to the native albumins and in part to the globu- 

 lins ; further, nucleins and a special keratin neurokeratin which 

 largely enters into the composition of the supporting structure of 

 the nerve-tissue, and is characteristic of this. Besides these various 

 substances nerve-tissue contains especially large amounts of so-called 

 myelins viz., lecithin, cholesterin, and protagon. A certain amount 

 of mineral salts and a large quantity of water constitute the remain- 

 ing known components of the tissue. 



As regards the distribution of these substances among the gangli- 

 onic cells and nerve-fibres more especially, our knowledge is as yet 

 quite incomplete ; but it appears from the analyses which are avail- 

 able that the gray substance of the brain in the dried state consists 

 to the extent of one-half at least of albumins, while the substances 

 which are soluble in ether amount to only about one-quarter of the 

 total quantity. Protagon is here present in only very small quantity. 

 In the white substance of the brain, on the other hand, it is found 

 in considerable amount, while the albumins constitute only one- 

 quarter of the dry material. In embryonic brains, in which the 

 medullary sheaths are not as yet developed, much smaller amounts 

 of lecithins are found than in the adult brain ; and it is noteworthy, 

 moreover, that protagon and neurokeratin are here both absent. It 

 may thus be concluded that these substances are essentially com- 

 ponents of the medullary nerve-fibres. According to Hoppe-Seyler, 

 indeed, the small amount of protagon which is found in the gray 

 substance of the brain is entirely referable to this source. The 



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