THE VISCERA OF THE HORSE. 357 



Eustachian valve, and only one anterior cava remains. The 

 aorta divides immediately after its origin into an anterior 

 and a posterior trunk; the latter becomes the thoracic 

 aorta; the former is the source of the arteries for the 

 head and the anterior extremities, giving off first the left 

 subclavian, and then as an " innominata" supplying the right 

 subclavian and the carotids. 



The trachea divides into only two bronchi, no accessory 

 bronchus being given off to the right lung. In the brain 

 the following points are worthy of notice. The medulla 

 oblongata presents corpora trapezoidea. The flocculi do 

 not project at the sides of the cerebellum, and the vermis 

 and lobes of the cerebellum are unsymmetrically convo- 

 luted. The cerebral hemispheres are elongated and sub- 

 cylindrical, and do not overlap the cerebeUum 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 pro- 

 tuberance) and the region which answers to the insula, are 

 not hidden by the overlapping of the convolutions in the 

 lateral aspect of the brain. The sylvian fissure is indi- 

 cated. The corpus callosum is large, and the anterior com- 

 missure is of moderate size. The posterior comu of the 

 lateral ventricle is wanting. 



Large air sacs are connected with the Eustachian tubes. 



The testes pass into a scrotum, but the inguinal canal 

 remains permanently open. 



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

 there is a large uterus masculinus. The large penis is 

 sheltered within a prepuce and is retracted by a special 

 muscle, which arises from the sacmm. 



The uterus is divided into two comu a, 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 sui'face of the chorion. 



