OF THE CERfcBKO-SPlNAL SYSTEM. 



Of the Functions of the Cerebro-spinal System of Nerves. 



I. General t'/ew of t fie J\"ervous System. At a former part of these notes we divided 

 the nervous system into two principal orders, viz the ganglial or vital, and the cerebro- 

 spinal. Of the former we remarked, that the globules of which it is constituted are disse- 

 minated in the structure of the Zoephyta, are organized into a homogeneous ganglion, 

 but imperfectly developed in many of the orders of the Ecliinvdennata, and are arrang- 

 ed into ganglia communicating by means of intermediate cords in the JlnneUdee, Cirrhi- 

 pedes, &c. The homogeneous nature of the ganglions disappears as the animal is pro- 

 vided with separate organs, especially with those devoted to the senses : and, with the 

 development of separate organs, accessary or subordinate, ganglions make their appear- 

 anee, which latter, in the progressive rise in the scale of the animal kingdom, assume in 

 the anterior part of the body of the animal the character of the rudiments of an ence- 

 phalon. So long as there exists only simple ganglia without any spinal cord, the gan. 

 glion representing the rudiments of an enoephalon, surrounds the oesophagus in the 

 manner of a ring. This encephalic ganglion is intimately connected with the ganglial 

 functions, and presides over those imperfect operations of sense with which the ani- 

 mal is endowed, and which are those more immediately subordinate to its fttnctions of 

 nutrition, and to its immediate preservation, 



In all animals possessing no other rudiments of the cerebro-spinal system, than an ac- 

 cessary ganglion disposed around the oesophagus, the manifestation of volition is by no 

 means distinct; their movements appear to be the result neither of reflection nor of 

 choice. An obscure instinct seems to be the actuating principle of those operations, 

 which may assume in them the nearest resemblance to those of volition. 



As we rise in the scale of the animal creation, and as we perceive the relation be- 

 tween the exterior world and the animal, to be more extended and intimate, owing to 

 the extension and perfection of the organs of sense and volition, so we perceive the ce- 

 rebro-spinal system more perfectly organized, more fully developed, and more compli- 

 cated in its structure. With the formation of the spinal cord, in the class of fishes,* 

 the accesary encephalic ring or ganglion disappears, and the encephalon is surrounded 

 by a protecting case, which is continued over the cord itself. 



The diversity and complication of the parts constituting the encephalon increase as 



mass composed of two symmetrical hemispheres, the prototype of the cerebral hemis- 

 pheres of all the superior classes. 



brain, as in that requiring the operation of trepan. In hydrocephalus, on the other 

 hand, where the fluidi s in large quantity, and there only remains the cortical part of 

 the brain, and pons Varolii, all the functions go on, and the memory can retain passages 

 of poetry. In one case, slight pressure upon the sinciput produced complete derange- 

 ment, and violent excesses of the passion of lust, which went off by removing, by tiie 

 trepan, the depressed bone. 



The veins being so minute, and being supplied with valves, perform in the opinion 

 of this physiologist, the office of absorbents which have never been observed in the 

 brain and carry the absorbed matter into the superior longitudinal sinus, which ap- 

 pears more a reservoir than a vein. 



The transparent mucus being not only one of the most abundant materials of which 

 the brain itself is composed, but also the medium by which the globules are kept to- 

 gether, and serving the same purpose in the nerves, Sir E. H. thinks that the commu- 

 nication of sensation and volition depends apon it. He concludes from all his experi- 

 ments a;id observations, that this fluid, as well as the principal materials of which the 

 body is composed, are met with in the blood. 



* M. Desmoulins has lately shown that many reptiles and several fishes offer not a 

 trace of grey substance in their spinal cords, and that on the contrary, this part is en- 

 tirely composed of white substance. He has also found that the sturgeon is entirely 

 \vithout a cerebellum ; and that its fourth ventricle possesses a considerable extent. 



He concludes that the dimensions and extreme development of the fourth ventricle 

 always coincides with the extreme development of the eighth pair of nerves. The cir- 

 cumstance of the grey substance being wanting in the spinal cord of some fishes mili- 

 tates against the opinion of M. OLlivier stated at a subsequent page. 



