372 THE NERVE-TISSUE. 



Carbon. Hydrogen. Nitrogen. Oxygen. Sulphur. 



Human hair .... 50.66 6.36 17.14 20.85 5.00 



Nails 51.00 6.94 17.51 21.85 280 



Horn 50.86 6.94 . . . . 3.30 



Egg-shell 49.78 6.56 16.43 22.90 4.25 



Turtle shell .... 54.89 6.94 16.77 19.56 2.22 



Neurokeratin .... 56.11-58.45 7.26-9.02 11.50-14.32 . . 1.63-2.24 



Its properties are the same as those of the keratins in general 

 (which see). 



THE MYELIN BODIES. 



In former years it was supposed that the medullary substance 

 of nerve-fibres consisted of a single substance, myelin, which was 

 characterized by the fact that on treating with water it formed 

 double-contoured droplets, which can readily be seen on microscopical 

 examination. According to Gad and Heymans, this myelin is in 

 reality lecithin in the free state or in loose combination, and we know 

 as a matter of fact that the peculiar reaction is due to decomposition- 

 products of such complex substances as protagon and certain com- 

 pound cholesterins. In speaking of myelin substances at the present 

 time we have reference to protagon s, lecithins, and cholesterins. 



Protagon. While there is evidence to show that different pro- 

 tagons exist, we are not as yet in a position to characterize such 

 forms individually, and for convenience 7 sake we shall speak of 

 protagons as a chemical unity at this place. The substance is not 

 strictly characteristic of nerve-tissue, as it has also been found in 

 other organs of the body, such as the spleen, in the stroma of the 

 red corpuscles, in pus, and in spermatozoa. But while it is here 

 present in only small amounts, it enters into the composition of 

 nerve-tissue to a considerable extent, and is thus quantitatively at 

 least peculiar to these structures. Whether or not the substance 

 occurs also in the gray matter appears doubtful, but in the white 

 matter and in the peripheral medullated nerve-fibres it is abundant. 



According to Liebreich, who was the first to isolate protagon 

 from brain-tissue, the substance has the composition C 116 H 24] N 4 PO 2 . 

 It is to be noted, however, that the elementary analysis of different 

 preparations has given rise to different results, which in itself sug- 

 gests the probability that different forms exist. According to some 

 observers, it also contains sulphur in molecular combination, but 

 recent investigations have shown that this is probably not the case. 



On decomposition with boiling baryta-water protagon yields the 

 same products as the lecithins, viz., fatty acids, glycerin-phosphoric 

 acid, and cholin. But, in addition, one or more glucosides are 

 obtained, which have been termed cerebrosides by Thudichum, and 

 of which three are now recognized. These are known as cerebrin, 

 kerasin,or homocerebrin, and encephalin. Others also may possibly 

 exist, and it is likely that the pyosin and pyogenin, which Kossel 

 and Freitag obtained from pus, belong to this order. 



On boiling with dilute mineral acids protagon also yields a re- 



