222 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



and new ones may originate in them ; so that there is no constant cor- 

 respondence between the number of fibres which enter a ganglion, and 

 of those which pass out of it. The only exception to the general fact, 

 that the vesicular matter occupies the centre of the ganglia, occurs in 

 the brain of Vertebrata, in which it is chiefly disposed on the exterior, 

 forming the cortical envelope. The reason for this variation is probably 

 to be found in the very large amount of this substance, which the brain 

 of the higher Vertebrata contains ; and in the necessity of the free 

 access of blood-vessels to it, which is provided for by a great extension 

 of its surface beneath the investing vascular membrane (pia mater), 

 more readily than it could be in any other mode. 



381. But the vesicular matter is not found in the central masses only 

 of the Nervous system ; for it presents itself also at those parts of the 

 surface or periphery which are peculiarly destined to receive the im- 

 pressions that are to be conveyed to the central organs. Thus on the 

 expansion of the optic nerve which forms the retina, there is a distinct 

 layer of ganglionic corpuscles or nerve-cells, with a minute plexus of 

 vessels, possessing all the essential characters of the vesicular substance 

 of the brain; and something of the same kind has been seen in con- 

 nexion with the corresponding expansion of the olfactive and auditory 

 nerves. Moreover, the study of the history of the development of these 

 organs has shown that the vesicular matter of the retina is an offshoot 

 (so to speak) from that of the optic ganglion, that of the labyrinth of 

 the ear being in like manner an offshoot from that of the auditory gan- 

 glion. Thus it is obvious that the fibres of the connecting nerve are 

 interposed between the cells of the peripheral and those of the central 

 organs, for the sake of preserving that connexion between them which 

 would otherwise have been interrupted ; and that the vesicular matter 

 is the active agent in the origination of those changes which take place 

 as a consequence of sensory impressions, whilst for the conduction of 

 such changes, the fibrous structure is alone required. 



382. The ultimate distribution of the nerve-fibres in the skin and 

 tongue, however, has not been so clearly made out, nor can their rela- 

 tion to cells be distinctly traced. These fibres are distributed, for the 

 most part, to the papillae, in which they can be frequently seen to form 



Fig. 72. Fig. 73. 



Capillary network at margin of lips. 



Distribution of the tactile nerves at the surface of the 

 lip ; as seen in a thin perpendicular section of the skin. 



loops (Fig. 72), accompanied by similar loops of blood-vessels (Fig. 73); 

 but no such loops can be seen in the fungiform papillae of the tongue, 



