770 77 IK NERVOUN NYNTEM. 



tinued into the lower layer of the velum interpositum. Just at the forward 

 bend this layer is really anterior to the other one. Posteriorly, this same layer is 

 in continuity with the pia mater covering the corpora quadrigemina, which, in its 

 turn, if traced ventrally, is seen to be continuous with that already mentioned 

 covering the crustae. 



It would appear, then, from the foregoing, that this whole arrangement of pia 

 mater is a complicated invagination or tucking-in process of an originally single 

 layer. Morphologically, however, we find this arrangement to be caused by the 

 absorption of the contiguous layers of the three " tubes " of pia mater already 

 referred to. Thus (cf. Figs. 459, 460, 461), the pia mater covering the crustae 

 should be considered, as it really does in an early stage of development, as run- 

 ning up on the outer side of, and around and behind the pulvinar of, the optic 

 thalamus to its upper surface, and thence inward to pass into continuity with the 

 lower layer of the velum interpositum, thus making one tube ; while, similarly, 

 the upper layer of the pia from the choroid plexus of the descending cornu should 

 be considered, not as being reflected mesially on to the crustoe, but as running 

 upward along the inner aspect of the internal capsule to the tsenia semicircularis, 

 and thence inward along the under surface of the floor of the body of the ventricle 

 to join with the upper layer of the velum interpositum, which, in its turn, should 

 be considered as splitting along its middle line, each half to bend upward, lying 

 mesial to the corresponding half of the fornix, septum lucidum, and corpus cal- 

 losum, to meet the corresponding layer of pia mater lining the mesial aspect of 

 the hemisphere above the corpus callosum ; thus forming two tubes. 



At the junction betAveen the choroid plexus of the body and that of the 

 descending cornu in the adult brain there is a twisting backward of the latter, so 

 that its free edge is directed posteriorly, while that of the former looks antero- 

 externally (Fig- 457). It may sometimes look mesially, bending over the fornix. 



Structure of Choroid Plexus. The plexus consists of minute and highly vas- 

 cular villous processes, composed of large round corpuscles, containing, besides a 

 central nucleus, several yellowish granules and fat-molecules, and covered by a 

 single layer of flattened epithelium. The arteries of the choroid plexus enter 

 along the descending cornu, and, after ramifying through its substance, send 

 branches into the substance of the brain. A constant branch, the anterior 

 choroid, enters at the extremity of the middle horn of the lateral ventricle, and 

 supplies the velum interpositum and the choroid plexus. The veins of the choroid 

 plexus terminate in the venae Galeni. 



The Transverse Fissure (Fig. 454). The descending cornu is a mere cleft ; 

 that is, its roof and floor are very close together. Hence the tcenia semicircular/'* 

 of the roof, which runs along in the substance of the white matter of the tem- 

 poral lobe, this white matter being the outer and under aspect of the beginning 

 of the internal capsule, is quite near the corpus fimbriatum in the floor. Between 

 the two pass the two layers of the pia mater which form the choroid plexus. If 

 this pia mater be pulled out, the epithelial inner wall will necessarily come with 

 it, and a cleft-like orifice into the cornu be produced. A similar cleft above will 

 be caused by removal of the velum interpositum and choroid plexuses of the 

 bodies of the ventricles, and if the plexus of the other cornu be removed also, 

 there will remain two large curved fissures, one on each side, extending from the 

 end of the descending cornu to the corresponding foramen of Monro. Begin- 

 ning at the foramen, the fissure will be bounded by edge of body and posterior 

 pillar of fornix above, and upper surface of optic thalamus beloiv (Fig. 462). At 

 the curve of the descending cornu the cleft will lie between pulvinar of optic 

 thalamus in front and edge of posterior pillar of fornix (now beginning to twist 

 into its position in the floor of the descending cornu as corpus fimbriatum) behind; 

 while along the cornu it is bounded below by corpus fimbriatum, and above by edge 

 of white matter of temporal lobe, along which is running the tcenia semicircularis. 

 These two fissures, taken together, are known as the transverse fissure of the brain, 

 and only exist when the pia mater and choroid plexuses are removed. Hence it 



