COMPOSITION OF THE NERVOUS SUBSTANCE 473 



cell, which is continuous with the sheath of the medullated fibres. This 

 membrane is nucleated and is composed mainly of a layer of very deli- 

 cate endothelium. 



In the gray matter of the nerve-centres, there is a finely granular 

 substance between the cells, which closely resembles the granular con- 

 tents of the cells themselves. In addition to this granular matter are 

 peculiar elements called by Robin myelocytes. These are found in the 

 cerebro-spinal centres, forming a layer near the boundary of the white 

 substance, particularly in the cerebellum. They exist in the form of 

 free nuclei and nucleated cells, the free nuclei being by far the more 

 abundant. The nuclei are rounded or ovoid, with strongly accentuated 

 borders, are unaffected by acetic acid, are finely granular and usually 

 are without nucleoli. The cells are rounded or slightly polyhedric, pale, 

 clear or very slightly granular, and contain bodies similar to the free 

 nuclei. The free nuclei are -g^Vo to ^-gVo ^ an ^ ncn (5 to 6 /A) in diam- 

 eter, and the cells measure 2 sVo^ to 2"oVo> an< ^ sometimes -j^Vo" ^ an mcn 

 (10, 12, and 15 /A). These elements are not to be confounded with 

 the so-called myelocytes found in bone-marrow. 



In the cerebro-spinal centres there is a delicate stroma of connective 

 tissue, chiefly in the form of stellate branching cells, which serves in 

 a measure to support the nervous elements. Neuroglia closely re- 

 sembles connective tissue, but the cells and their prolongations are 

 much finer. The cells are sometimes called glia-cells, or spider-cells. 

 Neuroglia, however, is derived from the epiblast and is composed of 

 neurokeratin, while the true connective tissues are derived from the 

 mesoblast. With the connective tissue, the neuroglia serves to support 

 the true nervous structures. 



The bloodvessels of the nerve-centres form a capillary network with 

 large meshes. The gray substance is richer in capillaries than the 

 white. A peculiarity of the vascular arrangement in the cerebro-spinal 

 centres has already been described in connection with the anatomy of 

 the lymphatic system. The bloodvessels here are surrounded by what 

 have been called perivascular canals. These are radicles of the lym- 

 phatic system. 



Composition of the Nervous Substance. The chemistry of the ner- 

 vous substance, so far as it is understood, throws little light on its 

 physiology. Certain proteids have been extracted which possess no 

 more than a purely chemical interest. The substance called cerebrin is 

 composed of carbon, hydrogen, oxygen and nitrogen, without either 

 sulphur or phosphorus. Protagon is a nitrogenous substance containing 

 phosphorus (Liebreich, 1865), and probably is a mixture of cerebrin 

 and lecithin. Lecithin is regarded as a nitrogenous fat. Other 



