i 



242 THE ANATOMY OP THE HOBSE. 



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 S^lvius,vfh.ioh is a canal tunnelled 

 beneath the corpora quadrigemina, and opening anteriorly into the 3rd 

 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 communicates 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 Jiih 

 ventricle. 



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

 anterior part of the roof of the 4:th 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 CEEBBEUM. 



Under the term cerebrum are included all the parts of the enoepha- 

 lon except the medulla, pons, and cerebellum. It forms a mass larger 3 

 than these taken together, although the amount by which it ex- 

 ceeds 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 Crtjra Cebbbri 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 boimdaries of a 

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

 front by the optic traets and commissure. Anteriorly each cruB 

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

 concealed by the optic tract. The crus is composed of a superficial and 

 a deep layer of nerve fibres with an intermediate thin stratum of grey 

 matter. The superficial layer of fibres is known as the crusta, and the 

 deep is termed the tegmentum. The fibres of both layers are continuous 

 posteriorly with the longitudinal fibres of the pons ; and they are trans- 

 mitted in front to the optic thalami, corpora striata, and grey matter 

 of the hemisphere. The corpora quadrigemina, which are superposed 



