LEPUS CUNICULUS 343 



passes round the front end of the corpus callosum and back for some 

 distance on the ventral surface. Each lobe sends back three roots 

 to the front end of the pyriform lobe. Of these two, a distinct 

 lateral root passing backwards and outwards, and a less distinct 

 median root running backwards and inwards to disappear in the 

 sagittal fissure, are discernible externally, while the third or inter- 

 mediate root is internal. Just behind the limit of the median roots 

 where they pass into the sagittal fissure is a small oval area, the 

 olfactory tubercle. Behind this is an area limited externally by 

 the inner margin of the lateral root, mesially by the sagittal fissure 

 and anteriorly by the olfactory tubercle. This is known as the 

 anterior perforated spot, because of the numerous branches arising 

 from the anterior cerebral artery that here perforate the surface of 

 the brain to supply its internal tissues. Posteriorly this area and 

 the sagittal fissure are terminated by the prominent optic chiasma, 

 from which arise the large optic nerves one on each side. The 

 nerves are represented only by short stumps, as they have to be cut 

 in removing the brain from the cranium. The chiasma is formed by 

 two large curved bands of fibres, the optic tracts, which come from 

 the base of the anterior quadrigeminal bodies and pass ventrally 

 and slightly forwards to unite in the middle line. 



Immediately behind the middle of the optic chiasma is a low 

 median elevation, the tuber cinereum, lying on the floor of the third 

 ventricle, the cavity of which is continued downwards as a short, 

 somewhat conical canal, the infundibulum, into the tuber. When 

 the brain is in situ, the fairly large oval pituitary body or hypophysis 

 cerebri, which lies in the sella turcica, is almost completely 

 separated from the brain by a double fold of the dura mater. It is 

 attached to the tuber, and the infundibular cavity passes on into 

 it. Generally in removing the brain from the cranium the pituitary 

 body is left behind so that its position is marked by a perforation 

 in the tuber. Behind the tuber is a second low eminence, the corpus 

 mammillare, which marks the hinder end of the floor of the third 

 ventricle and is divided into two corpora by a median groove in 

 some animals. If the pituitary body remains attached to the brain 

 it completely hides the corpus mammilare. 



This, then, completes the structures visible on the outside 

 of the fore-brain, and before passing on to the mid-brain it will be 

 as well to study the internal anatomy of the prosencephalon. As it 

 is a hollow structure we may start with the cavities. The lateral 

 ventricles are a pair of symmetrically arranged hollows, one in each 

 hemisphere, and as they arise as antero-lateral outgrowths of the 

 fore-brain vesicle of the embryo they are lined by the ependyma, 

 the characteristic epithelium that lines all the brain cavities and the 



