34 LECTURES ON THE CENTRAL NERVOUS SYSTEM. 



That part of the median hemisphere-wall which lies below 

 the corpus callosum (it appears behind it in the cut) is called 

 the septum pellucid um. That part of the original fissure be- 

 tween the hemispheres which remains between the right and 

 left layers of the septum is called the ventriculns septi pel- 

 lucidi. If you will imagine the corpus callosum removed from 

 Fig. 19, the continuation of the hemisphere wall into the septum 

 and the significance of the ventriculus will at once become 

 apparent. It is no real ventricle, but only that portion of the 

 fissure between the hemispheres which is covered in by the 

 corpus callosum. 



On each side of the fornix lies a passage from the third 

 ventricle to the two lateral ventricles, the foramen of Monro. 

 That part of the lateral ventricle which lies in the frontal lobe 

 is called the anterior horn, that which lies in the occipital lobe 

 the posterior horn, and that lying in the temporal lobe the in- 

 ferior horn. You can easilv insert vour fuller into each of these 



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horns. The basal regions of both hemispheres are connected 

 with each other by the anterior commissure. You ran see its 

 bundles of white medullary fibres passing in front of the pillars 

 of the fornix. 



The nucleus caudatus emerges from the floor of the lateral 

 ventricle. Farther back, however, parts come into view which 

 no longer belong to the hemisphere, the thalamus (inter-brain) 

 and the corpora quadrigemina (mid-brain). Behind these ap- 

 pears the upper surface of the cerebellum (roof of the hind- 

 brain). 



Along the whole inner border of the hemispheres runs the 

 thick mass of white fibres constituting the fornix. It arises 

 from the boundaries between the hemispheres and the mid-brain 

 on either side from the floor of the ventricle, and passes over the 

 optic thalamus into the apex of the temporal lobe. 



In removing the corpus callosum we removed also the 

 middle portion of this arch of the fornix ; so that we only see, 

 anteriorly, close to the septum -oellucidum, the ascending portion 



