466 GENERAL ANATOMY. 



are deeply seated: the extremities alone of these cords reach 

 to the two teguments of the surfaces of the body. 



745. The nervous system is formed of two substances, dis- 

 tinguished by their colour and their respective situation, into 

 white or medullary, and gray or cortical. 



746. The white nervous substance, called also medullary, 

 medullaris, because that for the most part it is enveloped by 

 the other, presents several shades of white. 



Its consistence varies a little in the different parts. It is in 

 general less elastic than gelatine, but a little more glutinous, 

 viscous or tenacious. The section is uniform in colour, and in 

 appearance homogeneous: red points or sanguineous striae are 

 alone perceptible in it. This substance is really very vascular; 

 when torn, the ruptured blood vessels become salient on its 

 unequal surface. 



The white nervous substance, plunged for some minutes in 

 boiling oil, or for some days in alcohol, in weak nitric or mu- 

 riatic acid, in acidulated alcohol, or in a solution of corrosive 

 sublimate, augments in consistence; and if it is then attempted 

 to distend or to tear it in any direction, a fibrous appearance is 

 perceptible. White filaments as fine as hairs can be separated 

 from it. The finest fibrils that can be obtained are so delicate 

 and so closely united with each other, that it is very difficult 

 to determine any thing relative to their length and the diame- 

 ter of the finest of them, or of the primitive fibrils. These 

 fibrils, parallel or concentric, are united in fascicles which 

 have, with respect to each other, different directions. It is 

 not known with certainty whether this fibrous disposition ex- 

 ists throughout the nervous system ; it has been found wherever 

 it has been sought for, and always the same in the same parts. 

 This fibrous structure is visible in some parts of the nervous 

 system, without any preparation; almost every where more 

 difficulty is experienced in tearing this substance in one direc- 

 tion than in another, and precisely in the direction which the 

 chemical preparations show to be that of the fibres. 



The white nervous substance, when dried, acquires a yel- 

 lowish colour and a corneous appearance; cut into thin slices, 



