fkE NERVOUS SYSTEM 1467 



and these transverse fibres constitute the eorpas eallosam. The fibres of the 

 tornix appear on the ventral aspect of the plate formed by the fused areas 

 of the cerebral hemispheres. On either side they are developed from the 

 hippocampus major, and they form, in succession, the fimbria, or tcsnia 

 hippocampi, the posterior pillar, one Judf of the body, and the anterior pillar, 

 of the fornix. The anterior pUlar passes downwards through the lamina 

 terminahs to the base of the brain, where it becomes twisted in the form of a 

 loop, and forms the white substance of the corresponding eorpas mammillare. 

 Thereafter its fibres terminate in the grey nucleus of that body, and from this 

 nucleus the bundle of Vieq d'Azyr passes upwards and backwards into the 

 optic thalamus. Each half of the fornix is therefore a commissure between 

 the hippocampus major and the optic thalamus of the same side. 



The hlppocampal eommissore appears on the back part of the ventral 

 aspect of the plate formed by the fused areas of the cerebral hemispheres. 

 Its fibres pass across from one hippocampus major to the other, and they 

 correspond to the region known as the lyra or psalterium. 



The anterior commissure is formed in connection with the dorsal part of 

 the lamina terminahs. over and anterior to which the mesial walls of the 

 cerebral hemispheres fuse. 



The posterior commissure is formed in connection with the back part of the 

 roof of the diencephalon, behind the pineal diverticulum. 



The so-called middle or grey eommissare is not a commissure properly so 

 called, but is formed by the fusion, over a limited area, of the grey matter 

 of the mesial surfaces of the optic thalami. 



The habenular commissure is produced by the decussating fibres of the 

 striae pineales. these fibres, as they decussate, forming the dorsal part of the 

 pedicle of the pineal body. 



Septum Lucidum and Ventricle of the Septum. — ^The anterior part of the 

 corpus callosum and its genu and rostrum are separated from the anterior 

 part of the fornix by a cleft-hke interval, which is a part of the great longi- 

 tudinal fissure. This interval is bounded on either side by a limited portion 

 of the thin mesial waU of the cerebral hemisphere. These portions of the 

 mesial walls of the cerebral hemispheres, known as the trapezoid plates, con- 

 stitute the t\%o laminap of the scptum lucidum, which septum forms a partition 

 between the adjacent portions of the lateral ventricles. The interval bet».veen 

 the two layers of tiie septum lucidum is called the ventricle of the septum, or 

 the fifth ventricle. It is destitute of an ependymal lining, and its cleft-like 

 space is entirely discoimected with the cephahc prolongation of the central 

 canal of the spinal cord. It therefore difiers from the ependymal ventricles 

 of the brain. As stated, it is an isolated portion of the great longitudinal fissure 

 of the brain. 



Lumen of the Neural Tube. — In the development of the spinal cord from 

 the spinal part of the neural tube it has been stated that the ventral portion 

 of the lumen (neural canal) of that part of the neural tube. ha\-ing become 

 modified, persists as the central canal of the spinal cord. The lumen of the 

 cephahc part of the neural tube, from which the various parts of the encephalon 

 are developed, also persists and remains in direct continuity with the central 

 canal of the spinal cord. In the cephahc part of the neural tube, however, 

 the limaen tmdergoes certain modifications. 



At the calamus scrip torius of the fotirth ventricle the central canal of the 

 spinal cord opens into the fourth ventricle, thus pertaining to the myelenee- 

 phalon and metencephalon. 



In the mesencephalon it becomes very narrow, jmd forms the aqueduct of 

 Sylvius, or iter a tertio ad quartum ventriculum, which leads from the fourth 

 to the third ventricle. 



In the diencephalon it forms the third ventricle. The third ventricle com- 

 municates by the foramina of Monro with the two lateral ventricles, which 

 belong to the telencephalon. 



The central canal of the spinal cord is thus in communication with all the 

 cerebral ventricles, properly so called. As previously stated, it is not related 

 to the ventricle of the septum (fifth ventricle), this not being an ependymal 



