This view of the connections of the corpus callosum would indicate it to be a commissure between the thalami and corpora striata, or betvveen the crura cerebri, as Tiedemaim sup posed, rather than between the hemispheres. Nothing is more difficult than the dissection of the fibres of the corpus adlosum beyond the intemal convolution : and it cannot be regarded as in any degree certain that the con nections of its fibres are liruited to those above described.
The developement of the corpus callosum in the fetus, 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 COMO 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. The anterior commissure may be reg,arded as truly a bond of connection betwecn 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 entile extent. Its situation in front of the anterior pillars of the fornix has been already des-el-died. 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 radiate extensively. It may be stated to con nect the convolutions of the middle lobes and the corpora striata.
3. The posterior commissure is a band of fibres, extended between the posterior extre mities of the optic thalami, upon which rests the base of the pineal body. Those fibres, which immediately support that body, have been distinguished as the pineal commissure ; but as they are evidently part of the same system as those which constitute the posterior commissure, there seems no good reason for separating them.
4. The sot t commissure is also extended be tween the thalami. It is composed of vesi cular matter with fibres, which pass from one side to the other. The intermixture of its fibres with vesicular matter distin guishes it from the other transverse commis sures already described. A layer of a simi lar nature connects the locus niger of each crus cerebri, and fills up the space between the crura—interpeduncular space. This has been already described as the pons Tarini, posterior perforated space. It consists of fibrous matter intermixed with vesicular, ex tended between the crura cerebri. It seems analogous to the soft commissure, and there fore entitled to be regarded as a commissure.
Of the manner in which the commissures connect the various parts between which they are placed, it is difficult to form an exact opinion. It is most probable that they form an intimate union with the grey matter of the segments which they serve to connect. It might also be conjectured that they are conti nuous with some of the fibres of the segments which they unite, or that they interlace with them in some intricate way, so as to come into intimate or frequent contact with them.
Tuber cinereum.—At the base of the brain we have already described a layer of pale grey matter which fills up the interval between the mamillary bodies and the optic commissure. It extends above the optic commissure for wards to the anterior reflexion of the corpus callosum, and forms intimate connections with the anterior pillars of the fornix, the optic tracts, the septum lucidum, and at the floor of the third ventricle with the optic thalami. It consists of vesicular matter with fibres, and resembles very much the soft commissure, to which it is very probably analogous in office.
The process called infividibulum or pituitary process extends frorn the inferior surface of the tuber cinereum down to the pituitary body. It is hollow, wide above, where it commu nicates with the third ventricle, and narrow below at the pituitary body. When cut across, fluid will escape from the third ventricle through it, and a probe passes readily from that cavity into it. It is composed of a layer of granules, derived, no doubt, from the epi thelial lining of the third ventricle, and some vesicular matter with bloochessels and fibrous tissue, which latter is derived from the pia rimier, and the special sheath of arachnoid reflected upon it.