626 



NERVOUS SYSTEM. (COMPARATIVE ANATOMY.) 



portional length is as 100 : 256. In the sheep, 

 as 100 : 232. In the deer, as 100 : 233. The 

 lateral lobes are very evident in all, and con- 

 volutions are observable on the external sur- 

 face, particularly in the horse. In the cat its 

 proportional length is as 100 : 200 ; in the 

 stoat as 100 : 228. The external convolutions 

 in both are numerous : in the monkey (Jig. 

 359, 1), the proportional length is as 100 : 305 ; 

 the laminae are numerous and small, thus ap- 

 proaching very much the characters of the same 

 part in man. 



The following is a table, shewing the actual 

 and relative lengths of the cerebral hemispheres 

 and the cerebellum in the Mammalia: 



On cutting into its substance in many of 

 these animals, the appearance of the arbor vitae 

 is more or less distinct, similar to the human 

 cerebellum. On its inferior surface is situated 

 its great commissure, the pons Varolii, which 

 first makes its appearance in this class of ani- 

 mals, and, with the exception of the transverse 

 fibres forming it being thinner and fewer in 

 number, particularly in those lower orders of 

 Mammalia where the cerebral hemispheres 

 were small, it presents but little differences from 

 the same part in the human adult brain. This 

 latter remark will equally apply to the fourth 

 ventricle, which has been an object of consi- 

 derable interest, and which, from being at first 

 an open exposed cavity, is now shut in and 

 concealed. 



[On the peculiarities of the brains of the 

 implacental class of Mammalia, see the ar- 

 ticles MARSUPIALIA and MONOTKEMATA.] 



On reviewing these statements of the nervous 

 system in the Mammalia, we observe that the 

 brain now preponderates greatly in bulk over 

 the spinal marrow ; this latter is also shorter, 

 and terminates by a true cauda equina. The 

 Jirst cerebral muss has now acquired its maxi- 

 mum of developement as regards size ; the two 

 portions of which it is composed are united by 

 a large commissure ; their exterior surface is 

 convoluted. The second cerebral mans is divi- 

 ded into two pairs of ganglia, in which the 

 cavities are obliterated. The third cerebral 

 mass has lateral hemispheres developed, striae 

 and convolutions on their exterior surface, and 

 an important commissure, the pons Varolii, on 

 its inferior surface. 



Having thus completed the investigations 

 proposed at the commencement of this paper, 



it may not in conclusion be without interest 

 and utility to take a very rapid review of the 

 developement of the nervous system in the five 

 large groups of animals in the system of ar- 

 rangement, as follows: 



a. The nervous system (perhaps) first exists 

 in a molecular form ; that is, it is made up of 

 globules dispersed throughout the homogeneous 

 texture of the animal, as in the Acnla, the 

 lower Entozoa, &c. 



b. This nervous matter arranged in a lon- 

 gitudinal direction forms filaments. The di- 

 rection which they assume is that of a ray, or 

 nerve, and a central point, or ganglion ; these 

 latter communicate with each other by commis- 

 sures, which unite them in the form of a ring. 

 This ring is situated around the oral orifice of 

 the animal ; it takes the name of the primary 

 nervous ring; and from it issue filaments in 

 a radiated manner, as in the Echinodermata. 



c. This oral primary nervous ring becomes 

 more complicated in itself; ganglions are first 

 developed on its lateral and inferior portions, 

 from which nerves pass off in a longitudinal 

 direction, as in the lower Mollusca, and 

 secondly on its superior surface, as in the 

 higher animals of this class : this superior 

 ganglion is at first proportionally small, as 

 in the Gasteropoda, but afterwards large, and 

 sometimes very large, as in the Cephalopoda. 

 It is the analogue of thetuberculaquadrigemina 

 of the higher animals. 



d. This primary nervous ring, in its most 

 highly developed form, now becomes repeated 

 several times in the body of the animal ; first, 

 in an undetermined number, as in the Helmin- 

 thoid Articulata; secondly, in a determined 

 number, as in the Entomoid Articulata. These 

 nervous rings are united by longitudinal com- 

 missures, and the most anterior one always has 

 a highly developed ganglion on its superior 

 surface. The uniting commissures possess 

 two distinct nervous tracts ; respiratory and 

 sympathetic nerves exist, as in the Insecta. 



e. These primary nervous rings are now 

 become ganglia (brain) ; the uniting commis- 

 sures are become primary nervous rings (spinal 

 marrow). First, the ganglia and their commis- 

 sures are nearly equally developed, and are 

 horizontal, as in the lower Vertebrata ; secondly, 

 the ganglionic formation predominates, and its 

 direction, with regard to the commissures, be- 

 comes more that of a right angle, as in the 

 higher Vertebrata; thirdly, the predominance 

 of the ganglionic formation is very much in- 

 creased, and its relative direction is that of a 

 complete right angle, as in the human species. 



(John Anderson.) 



NERVOUS CENTRES. (Human ana- 

 tomy). A nervous centre may be denned as a 

 mass composed of grey and white nervous 

 matter with which nerves are intimately con- 

 nected. In a physiological point of view it is 

 a centre of nervous action, as nerves appear 

 to conduct to it as well as from it. 



The nervous centres in the human subject 

 are the GANGLIONS, the SPINAL CORD, and the 

 BRAIN. 



