258 THE ANATOMY OF THE HORSE. 



from its apex backwards, when it will be seen to cover the optic 

 thalami and the pineal gland, and care must be taken lest the latter 

 be removed with it. 



The Optic Thalami. Each of these is a large grey-coloured body, 

 superposed to the crus cerebri behind the corpus striatum, and in front 

 of the corpora quadrigemina. Its upper surface is convex and covered 

 by the velum interpositum. When followed outwards this surface 

 changes its direction, looking backwards and downwards ; and it there 

 forms part of the boundary of the descending horn of the lateral 

 ventricle. Inwardly the right and left thalami are opposed to one 

 another along the middle line, and they include between them the 3rd 

 ventricle. In front each thalamus is separated from the corpus 

 striatum by a groove, in which will now be seen more distinctly the 

 taenia semicircularis. Behind, another groove isolates the thalamus 

 from the nates. 



The Pineal Gland is a small, reddish, conical body, named from its 

 resemblance to a pine cone. It stands by its base on the middle line 

 between the optic thalami and the nates. From its base two white 

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

 groove between the two thalami ; and at the foramen of Monro each 

 peduncle unites with the anterior pillar of the fornix to descend to the 

 base of the brain, and concur in forming the corpus albicans. In struc- 

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

 contains some branched corpuscles which are possibly nerve cells. In 

 man and some of the lower animals it also contains a quantity of gritty 

 calcareous matter termed the acervulus 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 Commissure 

 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) Commissure is 

 composed of delicate grey matter cementing the inner surfaces of 

 the thalami and apt to be more or less ruptured 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. 



