PONS VAROLII. 389 



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On the basis of the brain is a part called Tuber Annulare, or 

 Pans Varolii, which is formed by processes from the cerebrum, 

 and cerebellum ; it is in contact with the anterior and inferior 

 portion of the cerebellum in the middle. From this pa'rt the 

 medulla oblongata proceeds downwards and backwards, under 

 the cerebellum : and between the cerebelluni, the medulla ob- 

 longata, and the pons varolii, is the vacuity, called the fourth 

 ventricle of the brain. 



When the brain is in its natural situation, this cavity is below 

 and behind the nates and testes ; and from the cerebellum there 

 passes up to the testes, a lamen of medullary matter, which 

 closes it above. This lamen is called the Valve of Vieussens, 

 or the Valve of the Brain. Below, the ventricle is closed by 

 a membrane, which connects the medulla oblongata to the cere- 

 bellum. 



There is a passage into this cavity from the third ventricle, 

 which passes under the posterior commissure, the nates and 

 testes, entering the fourth ventricle below the testes. 



Of the Basis of the Brain, and the Nerves which proceed 



from it. 



When the brain is detached from the basis of the cranium, 

 and inverted, (which can be readily done, if the nerves that 

 proceed from it are divided, as it is inverted,) the tunica arach- 

 noidea appears more conspicuous on the basis than it is on the 

 upper part ; the pia mater is disposed round the convolutions in 

 the same manner that it is above ; but the nerves and vessels 

 connected with the surface of the brain are so much involved 

 with these membranes, that considerable dissection is required to 

 expose them properly. 



The anterior and middle lobes of the brain are very conspi- 

 cuous on the inverted surface. The anterior lobes appear separ- 

 ated from each other by the extension of the great fissure which 

 forms the two hemispheres. The middle lobes appear at some 

 distance from each other in the centre ; and the cerebellum forms 

 the posterior and most prominent part of the surface. 



When the brain has been carefully detached from the cranium, 

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