GENERAL CONFORMATION AND HISTOLOGY OF BRAIN. 



The nerve-fibres in the brain and spinal cord are of varying 

 size in adult mammals, and all are probably surrounded with 

 medullary sheaths. 



Every nerve-fibre, as it enters the central organ, loses its 

 sheath of Schwann. Only a thin layer, first seen by Ranvier, 

 and which is present even in peripheral nerves, is left to cover 

 the nerve-marrow after it enters the brain or spinal cord. 



In general, the parts which consist only of medullary fibres 

 appear white (white substance); those composed mainly of 



A,-- 



FIG. 25. 



Different nerve-fibres isolated from the spinal cord of a dog. ca, axis-cylinder ; nig, 

 medullary sheath ; g, peripheral membrane ; c, nucleus and protoplasm to be seen on the 

 surface of a few fibres. (After Rauvier.) 



neuroglia, axis-cylinder, and ganglion-cells, gray (gray matter). 

 The gray matter is more vascular than the white. During life 

 it exhibits, in the presence of sensitive reagents, a slightly acid 

 reaction. 



We owe our first accurate knowledge of the histology of the central nervous 

 system, as was stated in the first lecture, to Ehrenberg, Reinak, and Hannover. 

 After Hannover, Helmholz, in 1842, recognized in invertebrates the true relations 

 between ganglion-cells and nerve-fibres. In 1844, Kolliker discovered that a 

 double-contoured nerve might arise from a cell. In 1850 Rudolph Wagner dis- 



