78 
STRUCTURE OP GANGLIA.—NERVOUS ACTION. 
Fig. 24.—Thin slice of 
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 it is frequently called, 
is disposed on the surface of the brain; 
but it occupies 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 
present themselves also in certain situa¬ 
tions at the other extremities of the nerve- 
I" E Ganglia fibres. Thus we find a large proportion 
System, showing the of the retina (§ 535), which is commonly 
fibres ge amongst ne gIn- described as a mere expansion of the optic 
giionic ceils. 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 
Her YQ-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 use. 
The effects of such changes in the state of the Hervous 
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 
