THE NERVOUS SYSTEM. 518 



combustion it leaves behind an ash containing iron. 

 It is probably a metamorphosis-product of the coloring 

 matter of the blood. &quot;Whether the pigment in the rete 

 mucosum of the negro, and many pigments deposited 

 in diseases, are identical with it, is not decided. 



The vitreous humor and aqueous humor consist of 

 water with not quite 2 per cent, of solid substances 

 dissolved in it. In the vitreous humor these are 

 albumen^ sodium chloride, undetermined organic sub 

 stances, and urea ; the aqueous humor, on the other 

 hand, contains no albumen. 



The crystalline lens consists of concentric layers or 

 laminae, which are composed of compactly arranged, 

 clear fibres (probably tubes), and contain a very con 

 centrated liquid. This latter contains about 60 per 

 cent, of fat, cholesterin, and inorganic salts, and 35 

 per cent, of a protein compound, globulin (p. 490), very 

 similar to fibrinogenous substance. 



15. The Nervous System. 



&quot;Without entering here into a detailed consideration 

 of the fine structure of the brain, it may be remarked 

 in general, that the hemispheres of the cerebrum and 

 cerebellum consist of two masses differing essentially 

 from each other in construction and, without doubt, 

 also in composition. These are an outer gray layer, 

 the substantia cinerea, and a white fibrous mass, covered 

 by the former, the substantia medullaris. 



The gray matter is very abundantly supplied with 

 bloodvessels and poorly with brain fibres ; its principal 

 mass consists of peculiar microscopical globules. 



The marrow is less abundantly supplied with blood 

 vessels and water, and is very fibrous. Examined 

 under the microscope it is found to consist of very 

 delicate, transparent cylinders, formed of a thin mem 

 brane. They contain a semi-fluid, oily, clear mass, the 

 nerve-marrow. The white matter contains more fat 

 than the gray. In certain portions of the human brain 

 there have also been discovered microscopical bodies, 



