872 



THE NERVOUS SYSTEM 



as do the other regions of the pallium, but remain thin and constitute the septum pellucidum 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 ^\^dest portion lies in front, bounded by the genu and rostrum 

 of the corpus callosum, the rostral lamina, and the anterior pillars of the 

 fornix, to all of which it is attached. Prolonged backward under the body of the 

 corpus callosum, it narrows rapidly and terminates at the line of adherence be- 

 tween 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 pallium of the hemispheres. The laminae 

 enclose a narrow median cavity known as the fifth ventricle [cavum septi pellucidi]. 

 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. 



Fig. 685. 



-Diagram showing Some of the Principal Tracts and Synapses of the Ol- 

 factory Apparatus. 



Fornix 



Anterior commissure 

 Subcallosal gyrus 

 Parolfactory area 



Gyrus rectus ^ 



Perforating fibres 



J Medullary stria of thalamus 



, Gyrus cinguli 

 i Cingulum 



Olfactory tract 

 Olfactory bulb 



^ Longitudinal striae 

 on corpus callosum 



Hippocampal com- 

 missure (Lyre) 



Anterior thalamic 

 nucleus 



'■ Habenular nucleus 



. . Habenulo-pedun- 

 cular tract (fasci- 

 culus retroflexus) 



' Mammillo-mesen- 

 cephalic fasciculus 

 . . - -"^v Penduculo-tegmental 

 V tract 



y--'\ Interpeduncular nucleus 



\ Fimbria hippocampi 

 Mammillary body 

 Anterior perforated substance 



Olfactory epithelium 



Each 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 four varie- 

 ties 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 hippo- 

 campus 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 ol- 

 factory bulb; (3) the posterior angle of the septum pellucidum is preforated by some commissural 

 fibres passing from the body of the fornix and by some perforating fibres of the fornix, passing 

 from above through it to the fornix below; (4) anteriorly, some fibres from the longitudinal 

 8tri;p upon tlie corjius callosum pass through its inferior portion to join the fornix. 



The medullary stria of the thalamus [stria medullaris thalami] {striw pineaUs, taenia thalami),' 

 already described as to position, receives fibres from three sources, the majority at least of 

 which belong to the rhinencephalon: (1) fibres from the fornix near-by and tluis from the cor- 

 tex of hippocampal gyrus and gryus cinguli (a cortico-habenular tract) ; (2) filircs from the parol- 

 factory area and the anterior perforated substance, tlirough the septum pellucidum and lamina 

 terminalis (a more direct olfacto-habenular tract); (3) fibres arising from the cell-bodies in the 

 thalamus, supposedly chiefly from its anterior (olfactory) nucleus. These latter fibres make a 

 thalamo-habenular tract. 



The majority of the fibres of the medullary strife terminate in the habenular nuclei, situated 

 at the two sides of the stalk of tlio ojjiphysis. Most terminate in the lial)eiiular nucleus of the 

 same side. Some cross in the fuihrnnlar commissure (dorsal part of the jjosterior cerebral com- 

 missure) and terminate in the nucleus of the opposite side. A few are claimed to pass to the 

 nuclei of the quadrigeminate bodies and a few oth(;rs to join the association tracts of the mesen- 

 cephalon. AxoncH given oiT by the cells of the habenular nucleus curve anteriorly, inferiorly, 

 and then course posteriorly (fasciculus retroflexus) to terminate in the interpeduncular nucleus 



