INTRODUCTION. 85 



the anterior and posterior commisure connecting 

 them, and the intervening cavity is the third ven- 

 tricle : under the posterior part of this cavity, and 

 in the passage leading to the fourth, we find the 

 pineal gland, and, still lower, the four small emi- 

 nences called the tuhercula quadrigemina. The 

 cavity to which the name of the fourth ventricle has 

 been given, lies between the cerebellum and me- 

 dulla oblongata, and is, properly speaking, exterior 

 to the brain, being merely formed by its investing 

 membrane. It was before remarked, that the cere- 

 brum and cerebellum were separated from each other 

 by a membrane ; in the centre of this, however, there 

 is an opening through which the cerebrum sends off 

 two crura; these unite with two others, from the 

 cerebellum to form the tuber annulare, of which 

 the medulla oblongata is merely a prolongation, and 

 forms the commencement of the spinal marrow. 

 The nerves, which have their origin in the brain 

 and beginning of the spinal marrow, consist of me- 

 dullary matter, enveloped in a fibrous structure, and 

 pass through various apertures in the* base of the 

 cranium, to be distributed in the various organs of 

 sense and motion. 



In the proportions of the size of the brain relative 

 to the whole body, there are great differences in 

 mammalia, man being among those most abundantly 

 provided ; so, also, in the proportion between the 

 cerebrum and the cerebellum, man is, with the excep- 

 tion of one (callitrix sciurea) again the first. " But 



