498 



THE BRAIN 



they are continuous with the longitudinal fibres of the crura 

 and constitute a transverse commissure between the hippo- 

 campi of opposite sides. 



Dissection. The body of the fornix should now be divided transversely, 

 across its middle. Its posterior and anterior portions may then be raised 

 from the tela chorioidea of the third ventricle, and thrown apart from 

 each other. Had it been possible to raise the corpus callosum and fornix 

 together, the diverging crura of the fornix would have been seen to limit 



Cavum septi pellucidi 



\ 



Fornix divided s 



Vena , 

 terminalis'\> 



Genu of corpus callosum 



Septum pellucidum 

 _.- Caudate nucleus 



/f 1 Column 

 *sv"' of fornix 



& Tela chorioidea 

 JH-'" of third ventricle 



Internal cerebral vein 1 '' 



Cms of fornix '' 



Chorioid plexus 



""--..Body of fornix 

 reversed 



FIG. 204. Dissection to show the Tela Chorioidea of the Third Ventricle and 

 the parts in its vicinity. The fornix has been divided and thrown posteriorly. 



a triangular space on the under surface of the corpus callosum, anterior to 

 the posterior margin of the splenium. This interval is termed the lyra ; 

 it is traversed by a series of oblique markings which indicate the presence 

 of the transverse fibres passing across from one crus of the fornix to the 

 other. 



Tela Chorioidea Ventriculi Tertii (O.T. Velum Inter- 

 positum). This lamina is formed by a double layer of pia 

 mater which intervenes between the body of the fornix above, 

 and the roof of the third ventricle and the two thalami below. 

 Between the two layers are blood-vessels and some subarach- 



