

122 MAMMALIA. 



horse and ox, the anterior commissure, after leaving the anterior crus of the fornix, 

 also passes downwards and outwards, and divides into two portions, which approach a 

 group of low convolutions in the exterior of the brain in a similar situation, but not 

 covered by others ; one portion of it joins the tracts of the more anterior of the low 

 convolutions, the other joins the tracts of the same group near the middle of the 

 exterior surface of the brain. 



The thalamus varies in extent in different animals, in some degree according to 

 the size of the brain. Its surface is less extensive, in proportion to the optic nerve 

 and eye, in many animals, than in man. In the horse, which has a large brain, it 

 approaches that of man, but is shorter and sooner terminates in the optic tract ; it is 

 also narrower, and not so regularly oval ; anteriorly it receives the true visual tract. 

 It is connected with the anterior portion of the body of the fornix, with half of the 

 mammillary body, the optic commissure and the eminence surrounding the infundi- 

 bulum, and thus a thicker and more intimate combination is formed between those 

 several parts than that in man. The anterior portion of the thalamus is intimately 

 connected with the true visual tract ; the middle of its under surface is connected by 

 fibres with the involuntary tract ; its posterior surface then gains some increase from 

 the sensitive tract and from the quadrigeminal and geniculate bodies. It becomes 

 the optic tract which receives threads from the sensitive tract spread beneath the 

 epithelium in the inferior horn of the lateral ventricle, and is then continued to the 

 optic commissure. The two thalami are combined by grey fibres at the large rings 

 placed at the median sides, by which a communication is kept up similar to that by 

 the soft commissure in man. The posterior part of each thalamus is united by the 

 posterior commissure. 



The base of the brain is very different from that in man : in simiae, the anterior 

 lobes are very small in comparison with the middle ones, but they are separated by 

 the fissure of Sylvius ; in the porpoise there is also a separation. In many animals 

 there is no division into lobes, and very few convolutions are apparent. In simise, 

 the origin and size of the olfactory nerves correspond, in a considerable degree, with 

 the same in man, but in many instances the origin extends over a great part of the 



