120 MAMMALIA. 



upon the size of their superior parts, and on the same circumstance the depth at which 

 the great commissure is situated, as well as its dimensions. The structure of the great 

 commissure is nearly the same as in man, but when the brain is very small, it appears 

 on division almost like a line. In the porpoise it is thin, the raphe is distinct and par- 

 ticularly the transverse fibres. 



The dimensions of the lateral ventricle and the transparent septum vary with the 

 extent of the great commissure. The transparent septum is generally thicker and 

 narrower than in man, and very small in rodent animals. In some, as the horse and 

 sheep, the anterior horn is continued into the cavity of the great tubercle of the 

 olfactory nerve. In simise, in the place of the posterior horn, there are two small 

 angular excavations, and a very slight eminence connected with the superior and 

 posterior part of the great hippocampus. In simiee and the porpoise the body of the 

 fornix is thin and small, it is unattached and placed over the third ventricle. In many 

 instances the body of the fornix is much larger in proportion to the size of the 

 ventricles than in man, and then it conceals the thalami. In the horse and sheep its 

 anterior pillars are connected with the great commissure, and become inserted into the 

 anterior angle of each thalamus, and into the combination between the optic com- 

 missure, the eminence surrounding the infundibulum and the mammillary body; its 

 posterior pillars have not a loose edge, but become involved in the hippocampus. The 

 inferior horn generally winds downwards and forwards, as in man, and contains the 

 great hippocampus and the continuation of the choroid plexus. In the porpoise the 

 choroid plexus is flat, its inner border is a congeries of minute vessels ramified from 

 longitudinal ones running on the exterior border ; the plexus of each side communicates 

 underneath the fornix. The great hippocampus in simiae is continued down nearly as 

 far as the projecting point of the middle lobe, and appears to be solid, and combined 

 with the convolutions at the posterior and inner margin of the lobe, and is not seen at 

 the base of the brain, as in the horse, goat, and others, but in these its parietes are 

 thin, and connected in the ventricle with the inner surface of the prominence forming 

 the origin of the olfactory nerve, on the lowest part of which there are two small 

 eminences ; its concave or outer surface lies partly on the thalamus, and partly on the 



