1 194 



HUMAN ANATOMY. 



fore-brain, the later third ventricle, and contributes the anterior wall of this space. Attention 

 has been called to the invagination of the mesial pallial wall along the primary choroidal fissure 

 immediately above the line of attachment of the roof-plate to the hemisphere (Fig. 1031). The 

 latter is connected with its fellow of the opposite side by means of this thin lamina, upon 

 whose upper surface the mesoblastic sheet of the young pia is spread. On each side the 

 same sheet is prolonged through the choroidal fissure into the cavity within the pallium, 

 where it forms an extensive vascular mass, the choroid body, which, for a time, fills the 

 greater part of the hemispherical space, but from actual entrance into which it is now, as well as 

 subsequently, separated by the attenuated invaginated wall of the pallium. This displaced 

 wall, with the enclosed pial tissue, afterward becomes the choroid plexus of the lateral 

 ventricle and is carried downward along the mesial surface of the inferior horn with the for- 

 mation of the temporal lobe. Where the mesoblastic sheet overlies the roof of the fore-brain 

 it becomes the velum interpositum, which, it is evident, is continuous on each side with the 

 choroid plexus. Since the choroidal fissure begins in front at a point which later overlies the 

 foramen of Monro and, further, since the choroid ple.xuses of the two sides are connected by 



Fig. 1031. 



•Choroidal fissure 



Choroidal artery 



Pallium 



Roof of third ventricle 



Habenula 



Pineal body 



Thalamus 



Lamina terminalis 



Corpus striatum 

 Hypothalamic region 



Mid-brain 



Cavity of mid-brain 

 Isthmus 



Cerebellum 



Rhinencephalon' 



Optic recess 



Optic chiasm' 

 Infundibular recess / 



Maiumillary body' 



Pontine flexure 



Pons 

 Floor of IV ventricle 

 Medulla 



Cervical flexure 



Reconstruction of brain of human foetus of 3 months (50 mm.) ; mesial surface. X aVt.- Drawn from His model. 



the intervening velum interpositum, it follows that the plexuses converge towards and meet 

 over the foramina — a relation which they retain in the adult brain. The backward expansion 

 of the hemispheres is accompanied by a corresponding backward prolongation of the young 

 pia mater covering the roof of the diencephalon, later the third ventricle. After the corpus 

 callosum and the fornix have been superimposed, the impression is given from the relation of 

 the structures, as seen in the completed brain, that the pia has gained its position over the 

 roof of the thiid ventricle by growing forward beneath the splenium and fornix. That such, 

 however, is not the case is evident from the developmental history of the velum interposi- 

 tum. The secondary invagination of the brain-roof on each side along the median line by 

 the vascular tissue of the pia accounts for the production of the choroid plexus of the third 

 ventricle. 



The Cerebral Commissures. — The primary simplicity of the connections between the 

 hemispheres is disturbed by the formation of the commissures, which become necessary in 

 order to link together the increasing sheets of cortical gray matter. The development of 

 these commissures, the corpus callosum and the anterior commissure, as well as of the 

 septum lucidum, are intimately associated with changes which affect the lamina terminalis. 



