NERVOUS CENTRES. (HUMAN ANATOMY. THE ENCEPHALON.) 



701 



epiphysis upon the cms cevebri ; and in this 

 sense it may be classed with the striated bodies 

 and the quadrigemmal tubercles, which, with 

 the thalami, form a series of gangliform masses, 

 disposed in pairs, one beyond the other. 



The geuiculate bodies, although intimately 

 connected with the optic thalami, appear to be 

 distinct from them, but very similar in struc- 

 ture. A section made into the thalamus through 

 either of them shows a distinct line of demar- 

 cation between them. The optic tracts adhere 

 to the lower surface of each thalamus by their 

 inner margins, and when followed backwards 

 are found to form a very evident connection 

 with both the geniculate bodies. 



Corpora mamillaria. These bodies may be 

 conveniently noticed here, as forming part of 

 the series of gangliform masses in connection 

 with the brain. They are of a spherical shape, 

 covered on the exterior by very pure white 

 matter, which is apparently derived from the 

 anterior pillars of the fornix. When cut into, 

 they are found to consist of a mixture of vesi- 

 cular and fibrous matter, surrounded by a thin 

 cortex of the latter. Microscopic examination 

 proves this structure of the interior substance 

 to be of the same nature as that of ganglia, 

 and to resemble the optic thalamus. 



The fibrous matter is connected, at the upper 

 part of each body with the anterior pillar of 

 the fornix, and on the outside with a fascicu- 

 lus of fibres from the optic thalamus. It has 

 been supposed that these two bundles are con- 

 tinuous, and that the mamillary body results 

 from a twisting of the anterior pillar of the 

 fornix as it changes its direction to pass into 

 the substance of the thalamus. But the gan- 

 glionic structure of the mamillary bodies is 

 unfavourable to this view, and renders it more 

 probable that they are independent structures, 

 exercising proper functions as nervous centres ; 

 and the conslancy of these bodies in, at least, 

 the mammiferous series, increases this proba- 

 bility. 



Of the commissures of the brain. A large 

 number of fibres connected with the hemi- 

 spheres or the two gangliform bodies just de- 

 scribed, seem to serve the purpose of connect- 

 ing different parts, either on the same side of 

 the mesial plane, or on opposite sides of it. 

 Those on the same side constitute the longitu- 

 dinal commissures ; those on opposite sides the 

 transverse. 



Of the longitudinal commissures. These are 

 four in number. They all evidently belong to 

 the same system of fibres, separated from each 

 other by the developement of intermediate 

 parts. That which is on the highest plane is 

 the superior longitudinal commissure, which is 

 the fibrous matter of the internal convolution. 

 Internal and a little inferior to this is a se- 

 cond, very small, band of fibres, longitudinal 

 tract, (Vicq. d'Azyr,) which passes from be- 

 fore backwards along the middle of the corpus 

 callosum, parallel to the superior longitudinal 

 commissure, from which it is separated only 

 by the grey matter of the convolution. Both 

 these commissures are separated by the corpus 



callosum from a third, which takes a parallel 

 course to them, namely, the fornix, which 

 occupies a plane considerably inferior to both. 

 External to this and separated from it by the 

 ventricular projection of the optic thalamus, 

 we find a fourth band, which passes parallel to 

 the fornix : this is the teenia semicircular is. 



As these parts have been already described, 

 it will be unnecessary to do much more at pre- 

 sent than indicate the connections which they 

 serve to maintain. 



1. The superior longitudinal commissure con- 

 nects the convolutions of the inferior surface 

 of the anterior lobe with the hippocampus 

 major; and as its fibres pass above the corpus 

 callosum they form connections with some of 

 the other convolutions on the internal surface 

 of the hemisphere (Jig. 395). 



2. The longitudinal tracts of the corpus cal- 

 losum may be traced from about the same region 

 of the inferior surface of the anterior lobe as 

 the preceding commissure, near the perforated 

 space, and they pass backwards, winding over 

 the posterior reflection of the corpus callosum 

 to its inferior surface. 



3. The fornix* is, next to the corpus callo- 

 sum, the most extensive of the cerebral commis- 

 sures. That it consists of longitudinal fibres 

 cannot be doubted. Although commonly de- 

 scribed as a single structure united at the body 

 of the fornix, and spreading backwards and 

 forwards by its crura, it nevertheless isdistinctly 

 divisible along the middle line into two per- 

 fectly symmetrical portions. The adhesion of 

 the transverse fibres of the corpus callosum on 

 its upper surface, and of the terminal fibres of 

 its posterior reflection on its inferior surface 

 which form the lyra, is the principal bond of 

 union of these two lateral halves of the fornix. 



The separation of its anterior pillars in 

 front affords strong indication of its double 

 form. These pillars pass downwards in a 

 curved course, through the grey matter of 

 the tuber cinereum, to the mamillary bodies, 

 which are connected to the optic thalami by a 

 bundle of fibres which may be easily traced 

 into them. This bundle is described by Reil 

 as the rout of the anterior pillar of the fornix. 

 From the gangliform structure of the corpus 

 mamillare, I prefer to regard this band as a 

 medium of connection with the optic thalamus, 

 and to trace the anterior pillars to the ma- 

 millary body. 



The parts with which the fornix is connected 

 in front are the optic thalami, the mamillary bo- 

 dies, and the septum lucidum, which consists of 

 fibres having the same physiological import as 

 those of the corpus callosum, altered however in 

 direction by the backward position of the anterior 

 pillars, which adhere to the body of the fornix. 

 The tuber cinereum and the grey matter which 

 adheres to the lower half of the inner surface of 

 each optic thalamus, are connected with it ; 



* This commissure is called route a trois piliers by 

 the French, Triyone cerebrate, Chaussier : Vahts, 

 a-aifjio. TaTvioEiJEC of the Greeks. Mr. Solly has given 

 an excellent delineation of the fornix in his work 

 on the brain, pi. ix. 



