676 



NERVOUS SYSTEM. (NERVOUS CENTRES. THE EJJCEPHALON.) 



The horizontal portion of the fornix, as ex- 

 posed by this dissection, has the form of a 

 triangle, the apex of which is directed forwards, 

 and corresponds to the posterior angle of the 

 septum lucidum. Its base is situate behind, 

 and is enclosed by the posterior folded portion 

 of the corpus callosum. The apex is prolonged 

 into two rounded cords of fibrous matter, which 

 pass downwards and outwards, in a somewhat 

 curved course, with their convexity directed for- 

 wards. These are the anterior pillars of the 

 fornix. As they descend, they diverge from 

 each other. We can follow them down to the 

 base of the brain, where they form two small 

 tubercles, the corpora mamillaria, from which 

 fibres are continued upwards and outwards 

 into the substance of the optic thalamus. 



The posterior pillars of the fornix are ex- 

 pansions of fibrous matter which are continuous 

 with the angles of the base of its horizontal 

 portion. These bands are continued into the 

 lateral ventricle, and expand partly over the 

 posterior horn, and partly over the hippocampus 

 major in the inferior horn. The portion of the 

 fornix which is thus continued into the inferior 

 horn presents a fine concave edge directed in- 

 wards, which is the corpus Jimbriatum. 



It would thus appear that the fornix consists 

 of a horizontal triangular portion (corpus J'or- 

 nicisj resting on four pillars, which take some- 

 what of a curved course, and form numerous 

 connections with deep-seated and important 

 portions of the brain. The anterior pillars are 

 closely connected with the optic thalamus, 

 with the tuber cinereum, with the white matter 

 which forms the floor of the ventricle. The 

 posterior pillars are in intimate union with the 

 posterior and middle lobes of the brain. 



The fibres of the fornix are distinctly longi- 

 tudinal. So that, supposing it to be commis- 

 sural in its office, it may be stated to connect 

 the anterior lobe of the brain and the optic tha- 

 lamus with the posterior and middle lobes. 



The fornix is divisible into two equal and 

 symmetrical portions, one belonging to each 

 cerebral hemisphere. These portions are united, 

 as has been already stated, by the deep-seated 

 transverse fibres of the corpus callosum, and 

 by the terminal fibres of its posterior reflexion, 

 which form, on the inferior surface of the fornix, 

 a peculiar appearance called the lyra. The 

 transverse white fibres stand out in relief, cross- 

 ing at right angles the proper fibres of the 

 fornix. In many subjects, however, this ap- 

 pearance is but faintly indicated. 



The horizontal portion of the fornix rests 

 upon a triangular process of pia mater, which 

 is introduced into the interior of the brain, at 

 the fissure beneath the posterior reflexion of 

 the corpus callosum. This process is the velum 

 inter posit inn already described at page 635. 



The anterior pillars of the fornix bound in 

 front a space in which the velum interpositum 

 and choroid plexuses unite, and through which 

 the lateral ventricles communicate with each 

 other. This is fas foramen commune anterius, 

 described by the first Monro.* If a probe be 



* But previously recognised and described by 

 Vieussens. 



laid transversely in this orifice, it will have 

 above it the anterior extremity of the fornix, 

 in front the anterior pillars, and behind it the 

 point of junction of the three processes of pia 

 mater. 



Of the third ventricle. If the fornix be di- 

 vided transversely at about its middle, and the 

 segments reflected, and if the velum inter- 

 positum be removed, a fissure, the third ven- 

 tricle, is exposed, situate between the optic 

 thalami. This fissure extends forwards between 

 the anterior pillars of the fornix, where it is 

 limited by a band of white matter visible with- 

 out dissection in that interval. That band is 

 the anterior commissure, which lies just in front 

 of, and as a tangent to the convex border of 

 the anterior pillars of the fornix. 



At its posterior extremity the third ventricle 

 becomes very much contracted in all its dimen- 

 sions, and is continuous with a canal which 

 leads to the fourth ventricle (iter a tertio ad 

 quartum ventriculum, Aqueductus Sylvii). The 

 orifice of this canal is apparent at the posterior 

 extremity of the third ventricle, and is bounded 

 superiorly by a transverse cord of white matter, 

 the posterior commissure, which extends for a 

 short distance into the cerebral matter on either 

 side. The base of the pineal gland rests upon 

 this commissure. 



In this stage of the dissection, a general view 

 of the third ventricle is gained. This cavity 

 evidently results from the apposition of the 

 lateral halves of the brain proper, the parts 

 which more immediately correspond being the 

 inner surfaces of the optic thalami. The depth 

 of the ventricle corresponds, in a great degree, 

 to that of these bodies ; but it manifestly in- 

 creases towards the anterior extremity. Its floor 

 is formed by a layer of grey matter continued 

 from one side to the other, of the same nature 

 as that which has been already described as 

 covering the thalami. The deepest part of the 

 ventricle is aninfundibuliform depression, from 

 which the tubular process, seen at the base of the 

 brain (fig. 382, /), is continued down to the 

 pituitary body. Just beyond this part is the 

 anterior extremity of the ventricle, situate be- 

 tween the anterior pillars of the fornix and 

 behind the anterior commissure ; the depth of 

 which is much less than that of the infundi- 

 bulum. 



The floor of the third ventricle corresponds 

 to several parts of interest which have been 

 enumerated along the middle of the base of 

 the brain. Corresponding to the posterior ex- 

 tremity of the ventricle is the interval between 

 the crura cerebri, the pans Tarini, or inter- 

 peduncular space. Next in order, in the direction 

 from behind forwards, are the corpora mumil- 

 laria, which are succeeded by the tuber cinereum 

 and commissure of the optic nerves. The an- 

 terior extremity of the ventricle corresponds to 

 that portion of the tuber cinereum which ex- 

 tends between the optic commissure and the 

 anterior reflection of the corpus callosum. 



The roof of the third ventricle is formed by 

 the velum interpositum, already described as 

 giving support to the horizontal portion of the 

 fornix. 



