10 INTRODUCTION. 



marked by transverse furrows, which are parallel and contiguous. It is 

 of firmer consistence than the cerebrum, and is composed of cineritious 

 matter, through which the medullary substance ramifies in an arborescent 

 manner, so that, upon a longitudinal section being made, the medullary 

 matter appears like the stem and branches of a tree, trained regularly 

 against a wall. To this arrangement of the medullary matter the appel- 

 lation, arbor vitae, has been given. The cerebellum is divided into two 

 lobes, or hemispheres, and the medullary union, or commissure, between 

 them, is termed the vermiform process. At the inferior part of the cere- 

 bellum, where it rests on the commencement of the spinal chord, is a small 

 cavity, termed the fourth ventricle, at the bottom of which is an angular 

 impression, termed, from its resemblance to a writing pen, calamus scrip- 

 torius. This fourth ventricle is, in fact, a continuation of the third, with 

 which it communicates by a passage termed the aqueduct of Silvius. 



The commencement of the spinal chord (or rather, perhaps, its termi- 

 nation, before merging into the brain), while yet within the cranial cavity, 

 is formed by four crura, or medullary stems, two of which proceed from 

 the cerebrum, and two from the cerebellum. The cerebral crura arise 

 between the anterior and middle lobes ; those of the cerebellum from its 

 base, or trunk of the arbor vitae. The body, formed by the union of 

 these crura, is termed the medulla oblongata. Its line of separation from 

 the cerebrum is indicated by a transverse medullary band, striated trans- 

 versely, and termed the pons Varolii, or tuber annulare (bridge of Va- 

 rolius, or annulated eminence), which, of course, can only be seen by 

 viewing the brain in its basal aspect. 



Passing from the tuber annulare, the medulla oblongata is marked with 

 a longitudinal furrow down the middle, and with one on each side. Within 

 each lateral furrow is a slight eminence, termed corpus olivarium ; and 

 between the lateral furrows and the medial are two oblong elevations, 

 termed corpora pyramidalia. Invested with a continuation of the mem- 

 branes covering the brain, the medulla oblongata now enters the spinal 

 canal, marked, on its dorsal aspect, by a longitudinal furrow, and having 

 also a groove on each side the indications of its component fasciculi. 

 The precise influence of all the parts above enumerated, physiologists do 

 not pretend to explain. It is, however, ascertained, that, with the pre- 

 sence, or full development of certain parts, is associated mental elevation 

 in the scale of being ; while an incompleteness of union between the 

 different portions of the brain, the rudimentary condition, or absence 

 of certain parts, and the development of others, accompany mental, 

 and, it may be said, also, organic inferiority. To render the foregoing 

 description more intelligible, the following illustrations of the brain, in 

 different aspects, are annexed ; that representing its base, shewing the 

 origin of the nerves arising from it. 



