350 DISSECTION OF THE BABBIT 



iii. The velum interpositum is a thin membrane, 

 forming the roof of the third ventricle. On 

 its upper surface is a plexus of blood-vessels, 

 continuous with the choroid plexuses of the 

 lateral ventricles. 



iv. The pineal body is connected by a tubular stalk 

 with the hinder end of the velum interpo- 

 situm. 



Remove the velum interpositum, so as to open the third 

 ventricle. 



FIG. 44. Lepns cuniculus. Transverse section of the brain, passing 

 through the middle commissure of the third ventricle. The shading 

 indicates diagrammatically the distribution of white and grey matter. 

 The section is taken somewhat obliquely, so as to show different parts 

 on the two sides. (A. M. M.) 



A, optic thalamus, forming the side of the third ventricle. B, part of the 

 third ventricle above the middle commissure. C, middle commissure. D, in- 

 fundibulum. E, corpus callosum. P, temporal lobe of the left cerebral hemi- 

 sphere. G-, descending cornu of the right lateral ventricle. H, hippocampus 

 major. 



v. The third ventricle, or cavity of the fore-brain, 

 is a median vertical cleft between the optic 

 thalami. It is very narrow from side to side, 

 and communicates through the foramina of 

 Monro with the lateral ventricles. 



