78 STRUCTURY OF GANGLIA.—NERVOUS ACTION. 
filled with a finely-granular substance, which extends into 
their prolongations ; and in the warm-blooded Vertebrata they 
contain pigment-granules, which give them 
a reddish or yellowish-brown colour; so 
that the aggregations of vesicular substance — 
which we find in the larger nervous centres, 
are distinguishable by their greyish hue, 
This “grey matter,” as itis frequently called, 
is disposed on the surface of the brain; 
but it oceupies the interior of the spinal 
cord, and holds the same position in the 
smaller ganglionic centres (fig. 24). It 
is not only, however, in the central organs 
that nerve-vesicles are found; for they 
irs present themselves also in certain situa- 
Fig. 24.—Turx cutee or tions at the other extremities of the nerve- 
ONE OF een eANGtIA fibres. Thus we find a large proportion 
Sets, showing the of the retina (§ 535), which is commonly 
Fores’ amongst gan. described as a mere expansion of the optic 
 glionic cells. nerve, to be composed of nerve-vesicles 
that are scarcely distinguishable from those of the brain ; 
and it is probable that the ultimate branches of other sensory 
nerves have some such termination. Wherever we meet with | 
vesicular substance, we find it imbedded in a minute net- 
work of blood-vessels; and a copious supply of oxygenated 
blood is requisite to the due performance of its actions. 
62. There can be no doubt that the special office of the 
Nerve-fibres is to convey the influence of the changes which 
are effected in one part of the system, to other and remote 
parts ; just as the wires of a galvanic battery conduct the 
electric influence from the instrument which excites it, to 
some distant point where it is to be applied to some = 
The effects of such changes in the state of the Nervous. 
System are propagated in two opposite directions ;—the im- 
pressions made upon the skin and other parts possessed of © 
sensibility, being conveyed towards a portion of the nervous © 
centres called the sensorium, and there giving rise to sensa- 
tions;—and the influence of the emotions or volitions to 
which these sensations give rise (§ 7), being propagated from 
the central organs to the muscles, which they excite to con- 
traction. And by the discoveries of Sir C, Bell, hereafter to 
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