382 BASE OF THE BRAIN. 



vermiformis superior, and that \vhich is lodged within the vallecula being 

 the processus vermiformis inferior. The superior vermiform process is a 

 prominent longitudinal ridge, extending from the incisura anterior to the 

 incisura posterior cerebelli. In imitation of the hemispheres, it is divided 

 into lobes, of which three have received names, namely, the lobulus cen- 

 trails, which is a small lobe situated in the incisura anterior ; the monti- 

 culus cerebelli, a longer lobe, having its peak and declivity ; and a small 

 lobe near the incisura posterior, the commissura simplex. The lobes of 

 the inferior vermiform process are four in number, namely, the commis- 

 sura brevis, situated in the incisura posterior, below the horizontal fissure ; 

 the pyramid, a small, obtusely-pointed eminence ; a larger prominence, 

 the uvula, situated between the tonsils, and connected with them by means 

 of a commissure ; and in front of the uvula, the nodulus. In front of the 

 nodulus is a thin lamina of medullary substance, consisting of a central 

 and two lateral portions, the velum medullare posterius (valvula Tarini), 

 and between this velum in front, and the nodulus and uvula behind, is a 

 deep fossa which is known as the swallow^s nest (nidus hirundinis). The 

 velum medullare anterius is the valve of Vieussens, described with the 

 fourth ventricle ; both these vela proceed from the same point in the roof 

 of that ventricle, and separate from each other at an angle, the one passing 

 obliquely forwards, the other obliquely backwards. 



When a vertical incision is made into the cerebellum, that appearance 

 is seen which has been denominated arbor vitce cerebelli ; the white sub- 

 stance in the centre of such a section resembles the trunk of a tree, from 

 which branches are given off, and from the branches branchlets and leaves, 

 the two latter being coated by a moderately thick and uniform layer of 

 grey substance. If the incision be made somewhat nearer the commissure 

 than to the lateral border of the organ, a yellowish grey dentated line, en- 

 closing medullary substance traversed by the openings of numerous vessels, 

 will be seen in the centre of the white substance. This is the ganglion 

 of the cerebellum, the corpus rhomboideum or dentatum, from which the 

 peduncles of the cerebellum proceed. The grey line is dense and horny 

 in structure, and is the cut edge of a thin capsule, open towards the 

 medulla oblongata. 



The cerebellum is associated with the rest of the encephalon by means 

 of three pairs of rounded cords or peduncles, superior, middle, and infe- 

 rior. The superior peduncles, or processus e cerebello ad testes, proceed 

 from the cerebellum forwards and upwards to the testes, in which they are 

 lost. They form the anterior part of the lateral boundaries of the fourth 

 ventricle, and give attachment by their inner borders to the valve of 

 Vieussens, which is stretched between them. At their junction with the 

 testes they are crossed by the fourth pair of nerves. The middle pedun- 

 cles, or crura cerebelli ad pontem, the largest of the three, issue from the 

 cerebellum through the anterior extremity of the sulcus horizontal, and 

 are lost in the pons Varolii. The inferior peduncles, or crura ad medul- 

 lam obloTigatam, are the corpora restiformia which descend to the poste- 

 rior part of the medulla oblongata, and form the inferior portion of the 

 lateral boundaries of the fourth ventricle. 



BASE OF THE BRAIN. 



The student should now prepare to study the base of the brain : for this 

 ourpose the organ should be turned upon its incised surface ; and if the 



