CEREBRUM. 515 



the floor of the fourth ventricle as the fasciculi teretes ; they ascend to the deep or 

 cerebral part of the crus cerebri. 



Foville considers that a few fibres from each of the longitudinal tracts of the 

 medulla turn forwards, and are continuous with the transverse fibres of the pons. 



Septum. The pons is subdivided into two lateral halves by a median septum, 

 which extends through its posterior half. The septum consists of antero-posterior 

 and transverse fibres. The former are derived from the floor of the fourth ven- 

 tricle and from the transverse fibres of the pons, which bend backwards before 

 passing across to the opposite side. The latter are derived from the floor of the 

 fourth ventricle ; they pierce the longitudinal fibres, and are then continued across 

 from one to the other side of the medulla, piercing the antero-posterior fibres. 

 The two halves of the pons, in front, are connected together by transverse com- 

 missural fibres. 



CEREBRUM. 



UPPER SURFACE. 



The Cerebrum, in man, constitutes the largest portion of the encephalon. Its 

 upper surface is of an ovoidal form, broader behind than in front, convex in its 

 general outline, and divided into two lateral halves or hemispheres, right and left, 

 by the great longitudinal fissure. This fissure extends throughout the entire 

 length of the cerebrum in the middle line, reaching down to the base of the 

 brain in front and behind, but interrupted in the middle by a broad transverse 

 commissure of white matter, the corpus callosum, which connects the two hemi- 

 spheres together. This fissure lodges the falx cerebri, and indicates the original 

 development of the brain by two lateral halves. 



Each hemisphere presents an outer surface, which is convex, to correspond 

 with the vault of the cranium ; an inner surface, flattened, and in contact with the 

 opposite hemisphere, the two inner surfaces forming the sides of the longitudinal 

 fissure ; and an under surface or base, of more irregular form, which rests, in 

 front, on the anterior and middle fossae at the base of the skull, and, behind, upon 

 the tentorium. 



Convolutions. If the pia mater is removed with the forceps, the entire surface 

 of each hemisphere will present a number of convoluted eminences, the convolu- 

 tions, separated from each other by depressions (sulci) of various depths. The 

 outer surface of each convolution, as well as the sides and bottom of the sulci 

 between them, are composed of gray matter, which is here called the cortical sub- 

 stance. The interior of each convolution is composed of white matter, and white 

 fibres also blend with the gray matter at the sides and bottom of the sulci. By 

 this arrangement the convolutions are admirably adapted to increase the amount 

 of gray matter without occupying much additional space, and also afford a greater 

 extent of surface, for the fibres to terminate in it. On closer examination, how- 

 ever, the gray matter of the cortical substance is found subdivided into four 

 layers, two of which are composed of gray and two of white substance. The 

 most external is an outer white stratum, not equally thick over all parts of the 

 brain, being most marked on the convolutions in the longitudinal fissure and on 

 the under part of the brain, especially on the middle lobe, near the descending 

 horn of the lateral ventricle. Beneath the latter is a thick reddish gray lamina, 

 and then another thin white stratum ; lastly, a thin stratum of gray matter, which 

 lies in close contact with the white fibres of the hemispheres ; consequently white 

 and gray laminae alternate with one another in the gray matter of the convolu- 

 tions. In certain convolutions, however, the cortical substance consists of no less 

 than six layers, three gray and three white, an additional white stratum dividing 

 the most superficial gray one into two ; this is especially marked in those convo- 

 lutions which are situated near the corpus callosum. 



A perfect resemblance between the convolutions does not exist in all brains, 



