242 THE ANATOMY OF THE HORSE. 



extremity there is a miuute hole, which is the entrance to the short tube 

 that continnes the central canal of the cord into the posterior end 

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

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

 a \\Titing pen. The anterior end of the space lies under the valve of 

 Vieussens, and leads into the aqtieduct of Syhius, which, is acanal tunnelled 

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

 ventricle. The floor of the cavity is traversed by a longitudinal mesial 

 fuiTOw, 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 Jfth 

 ventAcle. 



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 veimiform process, and its lower face is free 

 and foiTos 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 cerebnim are included all the parts of the encepha- 

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

 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 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 foim 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 

 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 tenxied 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 



