ELEMENTARY FUNCTIONS OF NERVOUS STRUCTURE. 93 



which it is commonly found to be somewhat loosely suspended. The sheaths 

 of the several globules are connected with each other by prolonged filaments 

 or peduncles, as seen at BB, Fig. 9 ; and these form a kind of network, which 

 occupies the interstices of the fine vascular plexus, by which every part of the 

 gray matter is traversed. On the surface of the cortical substance of the brain, 

 another kind of structure is present, which is found, however, in smaller 

 quantity in other parts of the gray matter: this is a finely granular substance, 

 containing spherical or oval vesicles, with one or two dark granules in them. 

 In a rather deeper layer, these vesicles, instead of being irregularly scattered 

 through the granular substance, seem to have appropriated each to itself a 

 portion of the latter for an independent covering; and from this condition there 

 seems to be a regular gradation, till, in the yet depeer layers of the cortical 

 substance, the vesicles with their granular coverings, are replaced by perfect 

 ganglion-like globules with their filamentous sheaths. Hence it may be 

 surmised, that this substance is an incipient state of the true ganglionic matter. 

 11*2. It appears uncertain, from the results of the most recent microscopic 

 inquiries, whether the nervous fibres can be said to have any distinct termina- 

 tions, either in the ganglionic centres, or in the organs to which they are 

 distributed. In the gray matter of the brain of Vertebrata, they seem to form 

 a kind of plexus of loops, like that represented in Fig. 8, B ; but the ultimate 

 fibres never anastomoze. It Avould appear that, in the spinal cord of Verte- 

 brata, and in the ganglionic cord of the Articulata, the fibres pass through the 

 ganglionic substance, without any interruption ; but Mr. Newport has remarked 

 that they evidently become softer, and that their diameter increases, whilst 

 they are traversing it. (See 143 and Fig. 12.) The following appears to 

 be the mode in which the nervous fibres are distributed to the peripheral 

 organs. The trunks subdivide into small fasciculi, each of which consists of 

 from two to six fibres ; and these form plexuses, whose arrangement bears a 

 general resemblance to that of the elements of the tissue in which they are 

 placed. The primitive fibres then separate; and each, after passing over 

 several elementary parts of the containing tissue (as in the case of muscular 

 fibre), or after forming a single narrow loop (as in the sensory papillae), returns 

 to the same or to an adjoining plexus, and pursues* its way back to the nervous 

 centre from which it set out. In other words, each fibre becomes continuous 

 with another arising from the same or a neighbouring fasciculus. There is 

 thus, strictly speaking, no more termination of nerves, than there is of blood- 

 vessels; for both form circles. The characters of the fibres are scarcely altered 

 in the substance of the organs receiving them ; their sheaths become finer, but 

 they are npt lost; and there is no fusion of the nervous into the adjacent sub- 

 stance. The loops of the sensory nerves are surrounded, when they enter the 

 papillae, by a plexus of blood-vessels, including globules, which bear a close 

 resemblance to those of the ganglionic substance ; and each of these papilla 

 may probably be considered like the larger ganglia as a distinct originator 

 of nervous power ( 116). 



III. Elementary Functions of Nervous Structure. 



113. There can be little doubt that the functions of these two divisions of 

 the nervous system are different. That of the fibrous structure, as it exists in 

 the nervous trunks, is unquestionably to conduct or convey the influence of 

 changes, which have taken place elsewhere. And in accordance with what 

 has previously been stated, of the mode in which the mind is brought into 

 relation with the external world, through this nervous apparatus, we find that 

 there are (in the higher tribes of Animals, at least, if not in all) two sets of 

 fibres : one of which has for its office to convey external impressions towards 



