78 STRUCTURE 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 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- 

 Fig. 24. THIN SLICE OF tions at the other extremities of the nerve- 



E THE M,^HE"IC fibreS ' ThuS We filld a }S B P r POrtion 



SYSTEM, showing the of the retina ( 535), which is commonlv 

 flK* e aoiVt ne 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 

 'Neive-Jibres 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 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- 



