252 THE ANATOMY OF THE HORSE. 



bands — the peduncles of the pineal gland — extend forwards along tlie 

 gi'oove between the two thalami ; and at the foramen of Monro each 

 pednncle luiites with the anterior pillar of the fornix to descend to the base 

 of the brain, and concnr in forming the corpus albicans. In structure 

 the body presents some resemblance to lymphoid tissue, but it also 

 contains some branched corpuscles which are possibly nerve cells. 

 Imbedded in it is a quantity of gritty calcareous matter termed the 

 acet'vulus cerebri, or brain-sand. 



The Third Ventricle is a narrow space whose sides are formed by 

 the optic thalami. Its floor corresponds to the parts already examined 

 in the interpeduncular space, viz. , the pons Tarini, corpus albicans, and 

 tuber cinereum. Its roof is formed by the velum interpositum covered 

 by the fornix. In front it is bounded by the lamina cinerea, and it here 

 communicates with the lateral ventricles by the foramen of Monro. 

 Posteriorly the aqueduct of Sylvius enters it from the 4th ventricle. 

 The cavity is crossed by three commissures : 1. The Anterior Commis- 

 sure is a small white cord of nerve fibres stretching transversely between 

 the corpora striata at the anterior end of the cavity, and immediately in 

 front of the descending anterior pillars of the fornix. The fibres of the 

 commissure are traceable through the corpora striata into the white 

 matter of the hemispheres. 2. The Middle (soft) Cominissure is com- 

 posed of delicate grey matter cementing the inner surfaces of the thalami, 

 and apt to be more or less niptured in handling the brain. 3. The 

 Posterior Commissure is white, like the anterior ; and its fibres connect 

 the two thalami at the base of the pineal body, and immediately in front 

 of the nates. 



The 3rd ventricle has a ciliated lining continuous with that of the 

 4th throiigh the aqueduct of Sylvius, and with that of the lateral 

 ventricles through the foramen of Monro. In the foetiis the cavity com- 

 municates through the tuber cinereum and infundibuhim with tlic 

 pituitary body. 



The Foramen of Monro, or Foramen Commune Anterius, is the common 

 point of communication between the 3rd and lateral ventricles. It 

 might be desci-ibed as a short vertical shaft ascending from the fore- 

 part of the 3rd ventricle, and opening under the fornix, which is thrown 

 over it like an arch. Beneath this arch the lateral ventricles communi- 

 cate with one another and with the 3rd ventricle. 



The Corpora Quadrigemina are two pairs of bodies superposed to the 

 crura cerebri behind the optic thalami. The anterior pair of bodies, or 

 Tuites, are larger than the posterior pair, or testes, from which they are 

 separated by a groove. Between the right and left nates there is a well- 

 defined groove, but the groove between the testes is faint or not observ- 

 able. The nates are grey on their surface, but the testes are white. 

 The bodies were named nates and testes from a fancied resemblance 



