THE BRAIN AND ITS MEMBRANES. 



767 



above the inter-brain, is thus clear : (1) The pia lining the mesial aspect of each 

 hemisphere is absorbed by the formation of the corpus callosum and the coming 

 together of the two halves of the fornix ; hence (2), the part above the corpus cal- 

 losum becomes continuous with that of the other side across the upper surface of 

 the corpus callosum : while (3) the pia on the under surface of each hemisphere 

 (tif-lnir the corpus callosum and fornix) becomes continuous with that on the under 

 surface of the other, and forms one layer from side to side (upper layer of velum 

 interpositum I. This upper layer of the velum interpositum is now in close con- 

 tact with the layer of the pia covering the superior surface of the inter-brain 

 (lower layer of the velum interpositum), but they are not absorbed, because no 

 adhesions take place between the corresponding portions of the brain. 



(hi tli.'- */</,.> the effect of the adhesion between the hemisphere and the optic 

 thalarnus is to cause absorption of the layer of pia mater covering each; 



FIG. 459. Diagram of cross-section of hemisphere vesicles, inlet-brain, and mid-brain to explain formation 

 of velum interpositum and choroid plexuses. The red line is the pia mater. (B. B. G.) 



hence, as the pia mater on the hemisphere is originally continuous with the upper 

 1<i)j:-r of the velum interpositum, and the pia mater on the outer side of the optic 

 thalamus with the lower layer of the same, it follows that the two layers become 

 continuous at their margins or along the line where each is "cut off," as it were, 

 from its original prolongation (Figs. 460, 461). 



This in / >v/ ; " is at first ///'///// the border between the superior and external sur- 

 face of the thalamus, but soon becomes shifted mesially. so that it comes to lie 

 along the groove on the superior surface of the thalamus. This shifting is due 

 to the absorption of the pia-mater layers external to the groove, caused by the 

 adhesion which has taken place between the subjacent portion of the thalamus 

 and the superjacent portion (epithelial floor, see below) of the hemisphere. 



The anterior extremity of the velum interpositum, narrow and bifid, as already 

 described, is necessarily limited by the curve of the anterior pillars of the fornix, 

 behind which the two layers are continuous, because it is at and around this point, 

 which might be regarded as a sort of hinge, that the hemispheres swing up and 

 over the inter-brain, carrying with them each one half of the future upper layer 

 of the velum interpositum. 



