116 FUNCTIONS OF THE NERVOUS SYSTEM. 



Par Vagum is very intimate in Fishes, the class which approaches nearest in 

 the character of its nervous system to the Invertebrata. We may reasonably 

 infer, then, that the anterior ganglion is the principal centre of the reflex actions 

 of the nerves which correspond to the third branch of the fifth pair, to the 

 glosso-pharyngeal, and to the gastric portion of the Par Vagum, in Vertebrata ; 

 whilst the branches which connect them with the cephalic ganglia, bring these 

 nerves more or less under the influence of the latter. 



153. The lateral ganglia seem more analogous to the centres of the Sympa- 

 thetic system in Vertebrata; especially in the connection of their branches 

 with all the other systems of nerves; and in the share which they have in the 

 formation of the coeliac ganglia. This view of the relative functions of these 

 two divisions of the stomato-gastric system, is strengthened by the fact, that 

 the connection between the Sympathetic system of Fishes and the Par Vagum 

 is much more intimate than in the higher Vertebrata; although, even in the 

 latter, as will be shown hereafter, it is by no means so slight as it appears.* 



154. Upon taking a general review of the facts which have been stated, 

 and of the inferences which have been erected upon them, we perceive that 

 between the strictly sensorial functions of the nervous system and those ope- 

 rations in which its internuncial character only is employed, a tolerably distinct 

 line of demarkation may generally be drawn. We have hitherto viewed this 

 apparatus under two aspects:!. As the instrument of the mind, by which it 

 acquires a knowledge of the external world through the medium of sensation, 

 and operates upon it by an exercise of volition. 2. As the means by which 

 various movements are excited in the bodily structure, that are immediately 

 necessary to the performance of the organic functions, and to its protection 

 from injury; these motions take place in direct respondence to external im- 

 pressions, without the intervention of the will, without any designed adapta- 

 tion to purpose on the part of the animal, and often without its consciousness 

 being necessarily affected . The first of these objects appears to be answered, 

 chiefly if not entirely, by the cephalic ganglia and the nerves proceeding from 

 them. The second is carried into effect by the ganglia connected with each 

 organ, or series of organs, whose movements are thus excited. We have seen, 

 that, however small is the bulk of the cephalic ganglia compared with the 

 sum of the other masses, they send nerves to every part of the body supplied 

 by the latter; for the purpose, it would seem, of controlling, harmonizing, or 

 antagonizing their actions. These nerves proceed as connecting trunks from 

 the cephalic ganglia to the other centres ; and then divide into filaments, which 

 unite with those proceeding from them to the several organs. Each organ, 

 therefore, receives four sets of fibres; an afferent and efferent set, which con- 

 nect it with the cephalic ganglia, and are the channels of sensation, and of the 

 influence of the will; and an afferent and efferent set, which connect it with 

 its own peculiar ganglion, as well as with other similar ganglia ; and which 

 serve to convey the stimulus of impressions that produce motions by reflected 

 influence. In proportion as the special sensory organs are developed, and the 

 actions of the animal are less completely of a simply reflex character, we find 

 the cephalic ganglia and system of nerves more predominant. 



155. We observe among the Articulata the greatest perfection of instinctive 

 movements any where exhibited. In these movements there is a most remark- 



* The view given above of the comparative structure and offices of the Nervous Sys- 

 tem in the Invertebrated animals, is chiefly abridged from the Author's Prize Thesis on 

 this subject, in which additional details will be found, as well as many other illustrative 

 figures and references to authorities. He has there, also, discussed the physiological 

 explanation which had been previously given of the double nervous cord of the Arti- 

 culata; and having shown that it is neither consistent with itself, nor capable of being 

 applied to the other Invertebrata, he has deemed it unnecessary to complicate the pre- 

 sent sketch by introducing it. 



