176 Mammalia. 



two in number, run along its under surface, beginning at tbe 

 Foramen of Monro by tbe union of branches from the Yense 

 Corporis Striati and Yense Plexus Choroides (noted above). 

 The Yense Galeni unite posteriorly in a single trunk, which 

 terminates in the ^ Straight '* sinus. 



On separating the Corpus Fimbriatum from the Thalamus 

 Opticus, the position and boundaries of the great cleft at the 

 posterior part of the Brain, called the Transverse Fissure, will 

 be readily comprehended. This fissure is situated beneath 

 the Fornix, extending downwards, on each side, from the 

 Foramen of Monro in the middle line to the end of the des- 

 cending cornu ; its central part lies beneath the Fornix and 

 Corpus Callosum, and its lateral part is placed between the 

 edge of the Fornix, the Optic Thalamus, and Crus Cerebri. 

 Through this great slit, as above noted, the Pia Mater passes 

 into the Erain, and forms the Yelum Interpositum and 

 Choroid Plexus. 



6th Section. To Expose the Thalami Optici and Third 

 Yentricle. The Yelum Interpositum should now be raised 

 and removed ; the Hippocampus Major of one side dissected 

 away from subjacent structures and folded over on the other 

 side ; and the adjacent portions of Pia Mater removed : in 

 doing so the student must be especially careful at the posterior 

 border of the Yelum, where it invests the Pineal gland, so as 

 to leave that gland in situ. 



On the under surface of the Yelum Interpositum will be 

 noted two very short vascular fringes projecting into a narrow 

 oblong fissure which lies beneath and extends to the base of 

 the Brain called the Third Ventricle ; these are termed the 



* The * Straight ' Simcs, i.e. that which runs along the line of junciion of the 

 Falx Cerebri {i.e. the process of dura mater, which descends vertically in the 

 longitudinal fissure between the two hemispheres of the Cerebrum) with the 

 Tentorium {i.e. a horizontal process of dura mater between the Cerebrum and 

 Cerebellum), from the termination of the inferior longitudinal sinus (which is 

 contained in the lower /ree margin of the Falx Cerebri) back to the torcular 

 Herophili (or point where the longitudinal and lateral sinuses are confluent). 



