174 Mammalia, 



sists of two laminae, separated by a narrow interval, the 

 interval being called the Fifth Ventricle. 



3rd Section. To Expose the Fifth Yentricle. Tbe 

 space of the very minute fifth, ventricle * will come into view 

 by cutting across the part of the Corpus Callosum that re- 

 mains in the middle line ; detaching the anterior half from 

 the Septum Lucidum below, and reflecting it forwards. This 

 ventricle is in the sheep not more than iV*^. in length, and 

 close within the anterior fold or genu of the Corpus Callosum. 



4th Section. To Expose the Fornix. The posterior 

 half of the Corpus Callosum should now be detached with care 

 from the structures iminediately subjacent to it, and reflected 

 backwards ; and the Septum Lucidum should also be removed 

 by section along its inferior margin : by this means the 

 Fornix^ will be exposed, and seen to be a longitudinal lamella 

 of fibrous matter, triangular in shape, with the ap^x pointing 

 forwards : its free lateral edges (Corpora Fimbriata) form- 

 ing on each side part of the floor of the lateral ventricles ; and 

 the edges of its posterior crura being in contact with the 

 Choroid Plexuses : we have seen also that its upper surface is 

 connected in the median line to the Septum Lucidum in front, 

 and to the Corpus Callosum behind : lastly, at the apex it is 

 seen arching over the Foramen of Monro at a point which 

 will be shown later to be the front of the Optic Thalamus. 



The two openings (one on each side) known as the Foramina 

 of Monro, besides being the passage for the Choroid Plexus, 

 form also a transverse communication between the lateral 

 ventricles. 



5th Section. To Expose the under Surface of the 

 Fornix, the Yelum Interpositum, and the Transverse 

 Fissure of the Cerebrum. The Student should now divide 



Flower. PhU. Trans., 1865, p. 638. 

 t Fornix, an arch or Vault. 



