NERVOUS SYSTEM. (COMPARATIVE ANATOMY.) 



601 



longer, when they are observed lying out- 

 wardly between the deposited substance and 

 the cell-membrane, as in the muscles (c). The 

 remaining cavity appears to be filled by a pretty 

 consistent substance, the band of Kemak, and 

 discovered by him. In the adult a nerve, 

 consequently, consists, 1st, of an outer pale 

 thin cell-membrane the membrane of the ori- 

 ginal constituent cells, which becomes visible, 

 when the white substance is destroyed by de- 

 grees ((/) ; 2d, of a white fatty substance de- 

 posited on the inner aspect of the cell-mem- 

 brane, and of greater or less thickness ; 3d, of 

 a substance, which is frequently firm or con- 

 sistent, included within the cells, the bund of 

 Remak." 



From the '.description given in the foregoing 

 pages, we have seen that the prevailing anato- 

 mical element of nerves is a tube composed of 

 homogeneous membrane containing a soft pulpy 

 matter, the true nervous substance, divisible 

 into the white substance of Schwann and the 

 band of Remak, and that through the medium 

 of these nerve-tubes or fibres the grey matter 

 of the central masses is brought into connection 

 with the peripheral textures and organs. 



Whether the fibres which Henle has desig- 

 nated gelatinous fibres, which resemble very 

 much the central band of the nerve-tubes de- 

 prived of tubular membrane and white sub- 

 stance, perform a similar office, or whether they 

 serve to establish a connection between the grey 

 matter of the several nervous centres, are ques- 

 tions which we must leave for future considera- 

 tion. 



The sagacity of Galen long ago pointed out 

 that every part, which is capable of motion, 

 and at the same time possesses sensibility, 

 must receive two classes of nerves, motor and 

 sensitive. And it was reserved for the genius 

 of Bell in our own times to demonstrate that 

 the office of a nerve depends upon the powers 

 or endowments of its component fibres or 

 tubules, and that a nervous trunk may be 

 made up of fibres of different endowments 

 lying in juxta-position with each other. 



It is at the roots of the nerves that tubules 

 of distinct endowments are isolated from each 

 other. Thus Bell's experiments, which have 

 been confirmed by subsequent observations, 

 shewed that the anterior roots of spinal nerves 

 were motor, and the posterior sensitive; and 

 the determination of this important fact is the 

 foundation of all our knowledge of the phy- 

 siology of nerves. 



The difference in the powers or endowments 

 of the nerve-tubes does not appear to depend 

 upon any variety in their structure, or other 

 physical characters, (size perhaps excepted,) for 

 repeated examination has failed to detect any 

 such, but rather upon their peripheral and cen- 

 tral connexions. A sensitive nerve, while it is 

 organized at its periphery in such a manner as to 

 adapt it to the reception of impressions, must 

 be connected with that part of the brain whose 

 office it is to perceive the changes which such 

 impressions can produce. And a motor nerve 

 must be on the one hand connected with mus- 

 cular fibres, and on the other associated with 



such a part of the brain or other nervous centre 

 as is capable of exciting in it that change which 

 when communicated to a muscle will stimulate 

 it to contract. 



The precise mechanism of those nervous acts, 

 which 1 would distinguish as purely physical, 

 by reason of their independence of the mind, 

 is as yet unknown. It is still undetermined 

 whether a distinct series of fibres (excitomotory) 

 is necessary for them, or whether they may 

 not be performed by the same fibres which are 

 the channels of the mandates of the will, and 

 of the impressions of those stimuli which are 

 capable of producing sensation. 



( R. B. Todd.) 



NERVOUS SYSTEM, Comparative Ana- 

 tomy of.* In the following article it is in- 

 tended to describe the anatomy of the nervous 

 system in the different classes of animals as 

 they rise upwards in the scale. 



ACRITA. The class acrita consists of ani- 

 mals whose very characteristic is, that in them 

 the nervous system is molecular, consisting 

 of globules diffused through the cellular 

 tissue of their bodies. Amongst them we 

 distinguish, first, the Polygastricu, which are 

 minute microscopic animals, furnished with 

 numerous digestive cavities, in whom no ner- 

 vous filaments have as yet been traced ; still 

 they, many of them, possess eye-specks, they 

 show some indications of the sense of taste, 

 and perform their various motions in the diffe- 

 rent fluids as if under the well-directed gui- 

 dance of nervous power. These animals appear 

 as a punctiform homogeneous mass, in which 

 a nervous system does not as yet exist in a 

 distinct form : the nervous matter may be, 

 perhaps, every where diffused through the cel- 

 lular tissue of their body. These latter re- 

 marks will equally apply to the next class 

 the Porif'era ; of which the spongia officinalis 

 may be cited as an example. Its texture is 

 soft and gelatinous, and is probably made up 

 of nervous and muscular globules. 



POLYPIFERA. No nervous filaments have 

 been discovered, or described, in any of the 

 various forms and sizes of polypiferous ani- 

 mals, excepting in the genus actinia, respecting 

 which a doubt, almost amounting to a denial 

 of the statement, exists. The actinia may be 

 considered as an isolated polypus ; it has no 

 calcareous skeleton, and fixes itself to the rocks 

 by its fleshy base. Spix, a German anatomist 

 of high repute, gave plates of its nervous sys- 

 tem thirty years ago, and described it as con- 

 sisting of filaments with minute ganglia, sur- 

 rounding the fleshy base just mentioned, from 

 which were given off nerves to the different 

 parts. [Professor Rymer Jones believes he has 

 detected a delicate nervous thread, passing 

 round the roots of the tentacles, embedded in a 

 strong circular band of muscle, which sur- 

 rounds the orifice of the stomach.] Mr. Bell, 

 in dissecting several of these actinia, has not 

 been able to detect any nervous filaments ; 



* The Editor is responsible for the passages in* 

 eluded between brackets. 



