702 



NERVOUS SYSTEM. (NERVOUS CENTRES. THE ENCEPHALON.) 



and as its anterior pillars pass upwards in this 

 situation, they receive fibres from the neigh- 

 bouring convolutions. These pillars remain 

 separate from the mamillary bodies to the fo- 

 ramen of Monro, where they adhere to each 

 other and form the apex of the body of the 

 fornix. Traced backwards, the fibres of the 

 fornix pass into the posterior and inferior horns 

 of the lateral ventricle. In the former they 

 connect themselves with the hippocampus 

 minor by expanding over it, and in the latter 

 they spread over the hippocampus major, 

 forming the posterior pillar of the fornix, or 

 tenia hippocampi. 



The relation of the anterior pillars of the 

 fornix to the foramen commune anterius has 

 been already sufficiently described. Superiorly 

 the fornix adheres to the corpus callosum or to 

 the septum lucidum, and the anterior commis- 

 sure crosses in front of its anterior pillars, and 

 almost touches them. 



4. The fourth longitudinal commissure is the 

 tania semicircularis. It may be traced from 

 the corpus mamillare outwards and back- 

 wards in the groove between the optic thala- 

 mus and corpus striatum into the inferior 

 horn of the lateral ventricle, when its fibres 

 mingle with those of the middle lobe. It is 

 evidently part of the same system of fibres with 

 the fornix. 



Take away the corpus callosnm, the grey 

 matter of the internal convolution, the ventri- 

 cular prominence of the optic thalamus, and all 

 these commissures fall together and become 

 united as one and the same series of longitu- 

 dinal fibres. 



It is very remarkable how few fibres pass 

 between the great mass of the cerebrum and 

 the cerebellum. The processus cerebelli ud 

 testes are the only fibres which can be regarded 

 as forming commissures between these two seg- 

 ments of the encephalon, and they are of the 

 nature of longitudinal commissures. 



The transverse commissures are the corpus 

 callosum, the anterior commissure,t\\e posterior 

 commissure, the soft commissure. 



1. The corpus callosum, so called according 

 to some from the density of its tissue, is the 

 great commissure of the lateral halves of the 

 brain proper commissure, cerebri maxima of 

 Soemmering. The fasciculated character of 

 this structure is as obvious as that of any nerve 

 in the body, and the direction of the fibres is 

 clearly from one hemisphere to the other. 



From the description already given of the 

 corpus callosum, it is evident that its fibres 

 sink into the white substance of each hemisphere 

 above the level of the corpora striata, as well 

 as into that of the anterior and of the posterior 

 lobes. By its principal or horizontal portion 

 it connects the white matter of the lesser cen- 

 trum ovale of each side, and by the fibres which 

 form the anterior and posterior reflexions it 

 connects the anterior and posterior lobes. It 

 needs only a very superficial dissection to ascer- 

 tain thus much. 



To determine the precise fibres with which 

 those of the corpus callosum are continuous, 



and the relation which they bear to the lateral 

 ventricles, demands a much more minute dis- 

 section. This must be done, according to the 

 directions of Foville, who has given a most 

 elaborate description of this commissure, by 

 carefully separating it in the transverse direc- 

 tion from the internal convolution, on a har- 

 dened brain. Pursuing the dissection in this 

 direction, it may be detached from the sub- 

 stance of the hemispheres as far outwards as 

 the external border of the corpus callosum and 

 optic thalamus. Along this edge the fibres 

 curve downwards and inwards, and appear to 

 become continuous with some of those which 

 radiate from those bodies. The anterior and 

 posterior fibres enclose the anterior and pos- 

 terior horns of the lateral ventricles in radi- 

 ating forwards and backwards from the corpora 

 striata and optic thalami to those parts of these 

 cavities. 



This view of the connections of the corpus 

 callosum would indicate it to be a commissure 

 between the thalami and corpora striata, or 

 between the crura cerebri, as Tiedemann sup- 

 posed, rather than between the hemispheres. 

 Nothing is more difficult than the dissection 

 of the fibres of the corpus callosum beyond 

 the internal convolution : and it cannot be 

 regarded as in any degree certain that the con- 

 nections of its fibres are limited to those above 

 described. 



The developement of the corpus callosum 

 in the foetus, prior to that of the hemispheres 

 or convolutions, is favourable to the view of 

 its connections maintained by Tiedemann and 

 Foville. Comparative anatomy is, however, 

 more in accordance with the opinions of 

 Gall and Reil, that it is a commissure be- 

 tween the convolutions of opposite sides. It 

 exists only in those animals in which convo- 

 lutions are amply developed. In Fish, Reptiles, 

 and Birds it is absent, and the Mammalia with 

 least perfect brains, as the Rodents and Mar- 

 supialia. 



The corpus callosum is a stratum of con- 

 siderable thickness. Its fibres are situate on 

 different planes, which interlace with each 

 other so much as to render it impossible to 

 separate any layer for any distance, and the 

 difficulty is much increased as the fibres are 

 nearer the white mass of the hemispheres. 



2. Tlie anterior commissure may be regarded 

 as truly a bond of connection between the 

 hemispheres, as well as between the corpora 

 striata. It is a cylindrical cord of fibrous 

 matter, very definite in its course and con- 

 nections, and easily traced throughout its entire 

 extent. Its situation in front of the anterior 

 pillars of the fornix has been already described. 

 If followed on either side from this central 

 portion, it may be traced through the grey 

 matter of the anterior and inferior portion of 

 each corpus striatum into the fibrous matter 

 of each middle lobe of the brain. Its course 

 is curved with convexity directed forwards. 

 As it passes outwards on each side it becomes 

 flattened, and after it has traversed each corpus 

 striatum, it expands considerably and its fibres 



