CEEEBRAL COMMISSURES IN THE VEETEBRATA. 



461 



strand, each limb of wliicli ascends through the paratcrminal body and ultimately enters 

 the hippocampus : in tlic hippocampus its fibres are found scattering in the interval 

 between the regular column of cells and the lining epithelium of the lateral ventricle. 



The eommisstira ventralis, on the other hand, presents the aiij^earance of a transverse 

 horizontal baud extending laterally into the corpora striata. 



But in addition to all these structures the section passes through the recossus oj)(icus, 

 and hence opens up the third ventricle and the structures in relationship with its walls. 

 The walls of this portion of the third ventricle consist of the most cephalic portion of the 

 thalamic region. It may be noted here that in the region of the lamina terminalis 

 the anterior extremity of the thalamic region fuses not only with the corpus striatum, 

 but also with the corpus paraterminale. 



p<3r<3.T 



cv. 



Coronal sectiou. — Brain of Icttal Sjilx'awhjn (Dendy's stage R). 



A section (tig. 8) which passes immediately behind the lamina terminalis presents 

 many interesting points of difference when contrasted with the last figure. 



As in the last figure, the relations of " hippocampus " and the " neopallium " to the 

 coi'pus paraterminale and corpus striatum, respectively, are unchanged. Bat the 

 ventral relationships of the paraterminal bodies have undergone a notable change. The 

 third ventricle appears to have extended dorsally, and to have given off a lateral 

 diverticulum on each side. This lateral diverticulum joins the lateral ventricle, and 

 thus (in this section) completely separates the paraterminal body from the optic thalamus. 



This dorsal portion of the third ventricle, or centricnlKS communis, may be distino-uished 

 not inappropriately as the aula, a name introduced by Burt Wilder. The aula com- 

 municates on each side with the lateral ventricle by means of the foramen of Monro. 

 The roof of the aula consists of an irregularly folded jilate of cubical epithelium, and it 

 therefore diflPers very much from the relatively thick neuroglial plate which forms tlie 

 lamina terminalis. This roof, or tela, is attached on each side to the ventral lips of the 



