422 BASE OF THE BRAIN. 



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 appear- 

 ance is seen which has been denominated arbor vitce cerebelli; the 

 white substance 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 to the commissure than to the lateral border of the 

 organ, a yellowish grey dentated line enclosing 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 dentatuin, 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 ob- 

 longata. 



The cerebellum is associated with the rest of the encephalon by 

 means of three pairs of rounded cords or peduncles, superior, middle, 

 and inferior. The superior peduncles, or processus e cerebello ad iestes, 

 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 peduncles, or crura cerebelli ad pontem, the 

 largest of the three, issue from the cerebellum through the anterior 

 extremity of the sulcxis horizontalis, and are lost in the pons Varolii. 

 The inferior peduncles, or crura ad medullam oblonyatam, are the 

 corpora restiformia which descend to the posterior 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 purpose the organ should be turned upon its incised surface ; and 

 if the dissection have hitherto been conducted with care, he will find 

 the base perfectly uninjured. The arachnoid membrane, some parts 



