Different Animals, €¢. 479 
éxtremity of a nerve, which is attached to the 
brain, or spinal marrow, is usually named its 
origin® ; whilst that, which is distributed to 
the-organs of sense, muscles ‘&c, &c, is named 
simply its termination and by some writers its 
sentient extremity, when so disposed as to re- 
ceive impressions from external bodies, Of the: 
ganglions and plexuses of nerves it is not neces- 
sary to take notice in this place. 
The nervous system is composed of two prin- 
cipal swhstances,t named the cineritious, or cor- 
tical, and medullary; which differ considerably 
from each other in their colour, consistence, and 
other properties: The former being of a red- 
ish ash colour, semitransparent, softer, without 
distinct fibres, and possessing little, if any, sen- 
sibility: The latter being white or yellowish, 
opake, firmer, consisting .of very fine fibres vari- 
ously disposed and evidently possessing sensi- 
* Dr. Monro thinks this extremity is improperly so 
named and assigns a good reason, which will be noticed 
hereafter. 
_ + Besides these, there are two other substances noticed » 
by anatomists, namely, 1. The portio intermedia, which is 
whitish or yellowish, and is found in the cineritious 
substance of the posterior lobes of the cerebrum, 2. The 
portio nigra,which is found in the crura of the cerebrum. 
These substances are only found in the brain and are 
inconsiderable, Sce Soemmerring, T. IV. p. 47» 
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