850 THE NERVOUS SYSTEM 



The fornix proper is composed of longitudinally directed fibres, some of which, however, 

 cross the mid-line in the region of its body and course in the columns of the opposite side. For 

 the greater part, its fibres rise from the cells of the hippocampul gyri, but it is known to contain 

 some fibres which arise in the structures of the olfactory lobe proper and course through the 

 fornix to the hippocampal gyri. 



The anterior cerebral commissure is only in part concerned in the rhinenceph- 

 alon; it consists in greater part of cominissural fibres connecting; the two temporal 

 lobes. It forms one of the three commissures of the telencephalon, the other two 

 being the corpus callosum and the hippocampal commissure. It is a bundle of 

 white substance with a slightly twisted appearance, which crosses the mid-line in the 

 anterior boundary of the third ventricle between the lamina terminalis and the ante- 

 rior columns of the fornix (fig. 619), just below the interventricular foramen (fora- 

 mina of Monro). In each hemisphere its main or temporal portion passes out- 

 wards and slightly backwards beneath the head of the caudate nucleus and through 

 the anterior end of the lenticular nucleus, and thence is dispersed to the grey sub- 

 stance of the temporal lobe. It contains fibres both to and from the temporal lobe 

 of each side. In addition to these fibres the anterior commissure carries in its frontal 

 side two sets of fibres belonging to the olfactory apparatus: (1) fibres arising in the 

 olfactory bulb of one side, which pass by way of the medial olfactory striae through 

 it to the olfactory bulb of the opposite side ; (2) fibres which pass through it from the 

 medial stria (olfactory bulb) of one side to the uncus of the opposite side. 



The anterior commissure is a more primitive commissure than the corpus callosum, in that 

 it is present in the lower forms when the latter is absent, and diminishes in relative size and 

 importance as the corpus callosum appears and increa-ses in size. In man the appearance of the 

 anterior commissure precedes but little that of the corpus callosum. During the fifth month 

 the lamina terminalis, which then alone unites the anterior ends of the two hemispheres, developes 

 a thickening of its dorsal portion. In a part of this thickening, transverse fibres begin to appear 

 and their increase in number results in the partial separation posteriorly of the part containing 

 them from the rest of the lamina, and then follows the differentiation of this part into the anterior 

 commissure. The remainder of the thickening of the lamina continues to increase in size with 

 the increase of the hemispheres; its upper edge is directed posteriorly, and fibres begin to appear 

 in it which arise in the cortex of one side and cross over to that of the other side. These fibres 

 form the corpus callosum. 



The corpus callosum, a development of fibres in the upper, expanded portion of the lamina 

 terminalis, thus bridges over a portion of the longitudinal fissure between the hemispheres. In 

 the mean time, the fornix arises as two bundles of fibres, one from the hippocampus of each side. 

 In the complex mechanics of the development of the cerebrum these two bundles approach each 

 other under the corpus callosum, fuse for a certain distance, and together arch over the cavity of 

 the third ventricle and come to acquire their adult position. There results from these processes of 

 growth a completely enclosed space, a portion of the longitudinal fissure, the roof of which is the 

 corpus callosum, its floor, the body of the fornix, and its lateral walls, portions of the mesial 

 surfaces of the two cerebral hemispheres. The lateral walls of this space do not thicken as do 

 the other regions of the pallium, but remain thin and constitute the nrpliim pclliiriilum of the 

 adult, the space itself being the so-called fifth ventricle or cavity of the septum pellucidum. 



The septum pellucidum is a thin, approximately triangular, vertically placed 

 partition which separates the anterior portions of the two lateral ventricles from each 

 other. Its widest portion lies in front, bounded by the genu and rostrum of the cor- 

 pus callosum, the rostral lamina, and the anterior columns of the fornix, to all of which 

 it is attached. Prolonged backwards under the body of the corpus callosum, it nar- 

 rows rapidly and terminates at the line of adherence between the posterior portion 

 of the fornix and the splenium of the corpus callosum. It consists of two thin layers, 

 the laminae of the septum pellucidum, arrested developments of portions of the pal- 

 lium of the hemispheres. The laminse enclose a narrow median cavity known as the 

 fifth ventricle (cavity of the septum pellucidum) . This cavity is of very variable size, 

 is completely closed, and does not merit the term 'ventricle/ as applied to the 

 other cavities of the brain, in that it has no communication with the ventricular 

 system and has a different lining from the other ventricles. 



Kadi lamina of the septum pellucidum consists of a layer of degenerated grey substance next 

 to the fifth ventricle and a layer of white substance next to the lateral ventricle, the latter covered 

 by a layer of ependyma common to that ventricle. The white substance consists in part of fibres 

 belonging to the general association systems of the hemispheres, and in part of two varieties 

 of fibres concerned with the rhinencephalon: (1) fibres from each medial olfactory- stria are 

 known to reach the septum pellucidum and thence go by way of the fornix to the hippocampus 

 major; (2) fibres are thought to be contributed by the fornix to the septum pellucidum, and 

 through it reach the subcallosal gyrus and perhaps the parolfactory area and even the olfactory 

 bulb. 



