248 THE ANATOMY OF THE HORSE. 



by the valve of Vieussens, the under surface of the vermiform lobe, and 

 the reflection of pia mater from the medulla to the cerebellum ; laterally 

 it is bounded in its anterior third by the anterior peduncle of the cere- 

 bellum, and in its posterior two-thirds by the restiform body. The 

 widest part of the space is at the point where the peduncles enter the 

 cerebellum, and it contracts towards both extremities. At the posterior 

 extremity there is a minute hole, which is the entrance to the short 

 tube that continues the central canal of the cord into the posterior end 

 of the medulla. The pointed posterior end of the space is the calamus 

 scriptorius of human anatomy, so named from its resemblance to a 

 writing pen. The anterior end of the space lies under the valve of 

 Vieussens, and leads into the aqueduct of Sylvius, which is a canal 

 tunnelled beneath the corpora quadrigemiaa, and opening anteriorly 

 into the third ventricle. The floor of the cavity is traversed by a 

 longitudinal mesial furrow, and it shows the grey matter of the medulla 

 and pons. The cavity is lined by a ciliated epithelium, and it com- 

 municates by one or more minute apertures in its floor with the sub- 

 arachnoid space. On each side of the cavity, between the cerebellum 

 and the restiform body, there is a thickened piece of pia mater — the 

 choroid plexus of the Jfth ventricle. 



The Valve of Vieussens is a delicate, translucent fold, placed at the 

 anterior part of the roof of the 4th ventricle. The lateral edges of the 

 valve are fixed to the anterior cerebellar peduncles, its anterior edge is 

 attached behind the testes, and its posterior edge stretches across the 

 anterior vermiform process. The upper face of the valve is adherent to 

 the anterior vermiform process, and its lower face is free and forms the 

 anterior part of the roof of the 4th ventricle. The 4th nerve arises in 

 the valve, close behind the testes, the right and left nerves appearing 

 continuous with one another across the middle line. 



THE CEREBRUM. 



Under the term cerebrum are included all the parts of the encephalon 

 except the medulla, pons, and cerebellum. It forms a mass larger than 

 these taken together, although the amount by which it exceeds them is 

 much less in the horse than in man. The inferior aspect of the cerebral 

 mass is termed its base, and the student should begin by examining the 

 objects to be seen there (Plate 33). 



The Crura Cerebri are two thick, round, white cords, which appear 

 in front of the pons. At this point they are close together ; but as they 

 proceed forwards they diverge and form the posterior boundaries of a 

 lozenge-shaped area — the interpeduncular space, which is completed in 

 front by the optic tracts and commissure. Anteriorly each crus 

 disappears into the cerebral hemisphere, but its point of termination is 



