THE VISCERA OF THE HORSE. 305 



presents corpora trapezoidea. The flocculi do not project at 

 the sides of the cerebellum, and the vermis and lobes of the 

 cerebellum are unsymmetrically convoluted. The cerebral 

 hemispheres are elongated and suboylindrical, and do not 

 overlap the cerebellum when the brain is viewed from above, 

 The sulci are very deep, and separate numerous gyri, upon the 

 upper and outer surfaces of the hemispheres. The uncinate 

 gyrus (or natiform protuberance) and the region which answers 

 to the insula are not hidden by the overlapping of the con- 

 volutions in the lateral aspect of the brain. The Sylvian fis- 

 sure is indicated. The corpus callosum is large, and the an- 

 terior commissure is of moderate size. The posterior cornu 

 of the lateral ventricle is wanting. 



Large air-sacs are connected with the Eustachian tubes. 



The testes pass into a scrotum, but the unguinal canal re- 

 mains permanently open. 



The prostate is single. Cowper's glands are present, and 

 there is a large uterus masculinus. The large penis is shel- 

 tered within a prepuce and is retracted by a special muscle, 

 which arises from the sacrum. 



The uterus is divided into two cornua, and the vagina of 

 the virgin mare is provided with a hymen. The period of 

 gestation is eleven months. The yelk-sac of the foetus is 

 small and oval. The allantois spreads over the whole interior 

 of the chorion and covers the amnion, which is vascular. The 

 minute villi which it supplies with vessels are evenly scattered 

 over the whole surface of the chorion. 



The existing Equidae, are naturally restricted to Europe, 

 Asia, and Africa ; and are distinguished into the Horses, 

 which have homy patches on the inner sides of both pairs of 

 limbs — above the wrist in the fore-limb and on the inner side 

 of the metatarsus in the hind-limb; and the Asses, which 

 possess such callosities only on the fore-lirabs. 



Fossil remains oi EquidoR sx& abundant in the later ter- 

 tiary deposits of Europe, Asia, and the Americas ; but tlie 

 group is not known to be represented earlier than the miocene, 

 or later eocene, epoch. 



The EquidoB are among the very few groups of Mammalia, 

 the geological history of which is sufficiently well known, to 

 j)rove that the existing forms have resulted from the gradual 

 modification of very different ancestral types. The skeleton 

 of the older pliocene and newer miocene Mippnrion very 

 closely resembles that of an Ass, or a moderate-sized Horse. 

 There is a curious depression on the face in front of the orbit, 



