702 THE M:R VOL'S SYSTEM 



lucidum. Having reached the base of the Ijrain, they end in the gre}- nuclei of the 

 corpora albicantia. 



The anterior pillars of the fornix were formerly dL'scriljeil as twisting on themselves to form 

 bends or loops in the base of the brain, the corpora albicantia, the fibres alter forming the beiul 

 being eontinued ui)war(ls on the inner sides of the descending anterior jtillars as the bundles of 

 Vicq d'Azyr. This description is borne uut by naked eye dissection, but it has been shown that 

 the bundles of Vieq d'Azyr are discontinuous with the fornix fibres. (Forel and Gudden.) 



The septum lucidum is formed by tliiu portions of the heniisplicre walls, which 

 have become api)roximated and in part fused, but are separated in the greater i)art 

 of their extent by the fifth ventricle. It appears as two closely api)roximated thin 

 vertical laminie, separating the anterior cornua of the lateral ventricles of opposite 

 sides. It is attached above and in front to the under surface of the body and back 

 of the genu of the corpus callosum, below to the rostrum and to the basal white 

 connnissure, and behind to the anterior pillars of the fornix. Viewed from the 

 side, it is triangular in outline, with an acute angle directed backwards and occupy- 

 ing the narrow interval between the front of the body of the fornix and the corpus 

 callosum (fig. 412). 



Each of the two laminfe which constitute the septum lucidum contains three layers : a layer 

 of ependyma and epithelium next the lateral ventricle, a middle layer of white fibres, and a layer 

 of rudimentary grey matter next the fifth ventricle. A layer of connective tissue immediately 

 bounding the fifth ventricle (rudimentary pia-matral layer) is described by Testut. 



The fifth or Sylvian ventricle is a narrow, sagittally-directed, slit-like cavity 

 which originally formed a part of the great longitudin;d fissure. It lies between 

 the layers of the septum lucidum and it does not communicate Avith any of the 

 true ventricles of the brain. 



The great transverse fissure of the cerebrum is of a horse-shoe shape, and extends from the 

 foramen of Monro on each side to the termination of the descending cornu of the lateral ventricle. 

 It is not a fissure in the sense of forming a communication between the ventricles and the exterior 

 of the brain, but it may be considered as a fissure in the same sense that we use the term 'com- 

 plete fissure.' If we compare it with the adjacent dentate fissure, we find that the latter produces 

 a bulging (the hii)pocampus major) into the ventricular cavitj'. This bulging contains an inner 

 layer of white and an outer layer of grey matter, and is sim]>ly a fold of the ventiicular wall, 

 which is here thiimor than elsewhere. The floor of the dentate fissure is occu])ied by pia mater. 

 Now if we supi)ose the substance of the hipiK)campus thinned away until only the eiiithelial lining 

 of the ventricle was left, this epithelium would come in contact with the external pia mater, and 

 an arrangement resembling the choroid plexus, coming through a fissure, the hijipocampal fissure, 

 would be produced. The choroid plexus is therefore an internal convolution reduced to a layer of 

 epithelium forming the morphological wall of the ventricle, and produced by a fissure, the great 

 transverse fissure, which is occupied by a specialised and highly vascular part of the i)ia mater. 

 In comparing the great transverse fissure with the complete fissures of the cerebral hemispheres, 

 however, we must not lose sight of the fact that it is not limited to the hemis])here wall (exce))t 

 at its extremities in the temporal lobes), but is ])roduced, for the greater jiart of its extent, by the 

 folding of the |»rosencephalon over the thalamencephalon (page 670). According to Merkel and 

 Mierztyewsky, small slit-like ajiertures are found in the region of the transverse fissure near the 

 termination of the descending horn of the lateral ventricle. These place the cavity of the lateral 

 ventricle in communication with the subarachnoid space. 



Tlie velum interpositum, or tela choroidea superior, is a large triangular 

 fold of pia n)ater which overlies the third ventricle. The Ixxly and posterior jiillars 

 of the fornix and the splenium of the corpus callosum rest on its upi)er surface. It 

 is continuous laterally with tlie choroid plexuses of the lateral ventricles, which are 

 simi)ly the convoluted and highly vascular margins of the fold. Traced backwards, 

 the layers are seen to divaricate and to become continuous with the pia mater 

 covering the tentorial surface of the occipital lobes and the upjier surface of the 

 cerebellum. Two large veins, the veins of Galen, commence at the anterior 

 extremity of the velum interpositum. The veins of Galen are chiefly formed by 

 the veins of the corpora striata, and are continued backwards in the middle line 

 to end in the straight sinus. In this course they lie in a prolongation of the 

 subarachnoid space between the two layers of jiia mater which form the velum 

 interpositum, and usually unite into one vein before entering the straight sinus. 



