158 OUTLINES OF EQUINE ANATOMY. 



the cavities of tlie lateral ventricles but also extends down 

 to the base of the brain, where it forms the cavity of the 

 tuber cinereum, and the inf undibulum, and extends as far as 

 the pituitary body. Anteriorly it is bounded by the anterior 

 tvJiite commissure, which connects the two corpora striata 

 together. Posteriorly situated to the grey commissure is 

 the posterior commu7iicating foramen, which forms the com- 

 mencement of the iter a tertio ad quartum ventriculum. 

 Just above this is a small black cone-shaped body, the 

 pineal gland, the use of which is unknown — we sometimes 

 find it much enlarged. It contains pigmentaiy matter and 

 vessels arranged in an areolar stroma, and from it forwards 

 along the inner margins of the thalamic optici, above the 

 grey commissure, run the crura of the pineal body, which 

 anteriorly blend with the substance of the thalami. The 

 third ventricle is the cavity through which the grey com- 

 missure passes " like a transverse beam." Anteriorly, it is 

 bounded by the anterior white commissure, superiorly by 

 the velum interpositum, laterally by the thalami optici 

 and the crura of the pineal body. Inferiorly by the upper 

 surface of certain structures situated at the base of the 

 brain — tuber cinereum, corpus albicans, and locus perfora- 

 tus posticus. It presents three openings : inferiorly, the 

 infundibular ; sup ero- anteriorly, the anterior communi- 

 cating foramen ; supero-posteriorly, the posterior communi- 

 cating foramen. Its posterior boundary is the posterior 

 white commissure, which connects together the corpora qua- 

 drigemina, and forms the roof of the iter a tertio ad quartum 

 ventriculum. The corpora quadrigemina are four bodies 

 posteriorly placed to the pineal body. The anterior pair is 

 the roundest, largest, and darkest, and is termed the nates, 

 the posterior, or testes, more ovoid in form, posteriorly receive 

 tracts of white matter running from the cerebellum, iter 

 e cerehello ad testes. These tracts are connected together by a 

 thin membraniform layer of matter, valve of Yieussens, from 

 which the fourth pair of nerves arise. They circumscribe 

 anteriorly the fourth ventricle, "a boat-shaped cavity," 

 situated at the upper part of the medulla oblongata. Its 

 roof is formed anteriorly by the valve of Yieussens, cen- 

 trally by the under surface of the cerebellum, posteriorly 

 by the arachnoidean valve. Its floor and lateral walls are 

 formed by the pillars of the medulla oblongata, anteriorly 

 its cavity is continuous through the aquaduct of Sylvius or 



