698 



NERVOUS SYSTEM. (NERVOUS CENTRES. THE ENCEPHALON.) 



its characters in different brains, according to 

 the degree of tortuosity it exhibits, and the 

 number of small fissures which are met with in 

 it. The small folds which connect it with other 

 convolutions on the inner surface of the hemi- 

 sphere vary in number, and are generally found 

 most numerous at its posterior part. Some of 

 these folds are not distinctly visible unless the 

 sulcus above it has been freely opened, as they 

 are situated quite on its floor. 



2. The convolution of the Sylvian fissure. 

 This convolution forms the immediate boun- 

 dary of this great fissure. We have seen its 

 early developement in the simple brain of the 

 fox, and we may observe it gradually rising in 

 complexity through all the intermediate stages 

 up to the most highly developed brains. In 

 the elephant it is remarkably tortuous, and is 

 connected anteriorly as well as posteriorly with 

 convolutions which pass to the anterior and su- 

 perior and to the posterior part of the brain.* 

 In man it is also very tortuous, and the nume- 

 rous folds which pass from it forwards or back- 

 wards, forming primary or secondary convolu- 

 tions, render it difficult to isolate it sufficiently 

 for the anatomist to follow it throughout its entire 

 course. Its inner border is likewise interrupted 

 by the connections which it forms with the con- 

 volutions of the floor of the Sylvian fissure. 



3. Within the fissure of Sylvius we find that 

 remarkable group of convolutions called by Reil 

 insula, the island. It consists of a series of 

 small folds radiating from a common centre and 

 connected with the convolution last described 

 by still smaller folds, which cannot be seen 

 unless when the fissure has been very freely laid 

 open. The centre from which the convolutions 

 radiate is the apex of a cone, the base of which 

 adheres to the floor of the fissure. 



4. On the inferior surface of the anterior 

 lobe there is a pair of longitudinal convolutions 

 which enclose between them the fissure of the 

 olfactory process. The external of these con- 

 volutions is continuous with the convolution of 

 the Sylvian fissure. 



The numerous secondary convolutions which 

 are found over the surfaces of the brain render 

 it difficult to distinguish the primary ones. 

 These latter are indicated hy the antero-poste- 

 rior course which they take the former being 

 more or less vertical. The largest and most 

 tortuous convolutions are found on that part of 

 the external surface which corresponds to the 

 parietal bone. Next to them, in point of size, 

 are the convolutions of the anterior lobe, but 

 the smallest of all are those of the posterior 

 lobes. 



The hippocampi, major and minor, are 

 constant convolutions, which project into the 

 lateral ventricles, the latter into its posterior, 

 the former into its descending horn. 



In general the constituent fibres of the white 

 matter of the convolutions converge from the 

 inner surface of the cortical layer to the cen- 

 trum ovale, or if followed from the centrum 

 ovale, they radiate to the grey surface, whether 



* See Leuret, pi. \iv. re-presenting the external 

 surface of the elephant's brain. 



of a convolution or of a sulcus. A remarkable 

 exception is in the case of the internal convo- 

 lution, the fibrous matter of which constitutes, 

 as has been already explained, a longitudinal 

 commissure. The thickness of the cortical 

 layer is pretty uniform, at least relatively to 

 the size of the folds themselves. Throughout 

 its entire thickness it is mixed with fibres, which 

 are most numerous at its adherent surface, 

 but extremely few and scattered at its free 

 surface. 



In hydrocephalus the convolutions disap- 

 pear. The fibrous matter becomes greatly ex- 

 panded by the fluid accumulated in the ven- 

 tricles, and when its expansion has gone so far 

 as to equal the grey surface, the folded cha- 

 racter of the latter disappears. This takes 

 place precisely in the same way that the rugas 

 of the contracted stomach (as before referred to) 

 become obliterated when the muscular coat re- 

 laxes and allows the full distension of the organ. 

 Mayo supposes that other fibres are found in 

 the convolutions besides those which are conti- 

 nued into the centrum ovale. These are com- 

 missural ones, which pass from convolution to 

 convolution either between adjacent or dis- 

 tant ones : forming arches the convexities of 

 which are directed to the centrum ovale. I 

 have never succeeded in satisfying myself of 

 the existence of such fibres either in the fresh 

 brain or in that preserved in spirit. If they 

 exist, it is evident that they must be commis- 

 sural between particular convolutions. The 

 same anatomist supposes that similar commis- 

 sural fibres connect the laminae of the cere- 

 bellum. 



The principal bulk of the hemispheres is 

 formed by fibrous substance. This is shown 

 by the horizontal section which displays the 

 centrum ovale. These fibres radiate from those 

 surfaces of the optic thalami and corpora striata 

 which are in contact with the substance of the 

 hemisphere. Most of the fibres which emerge 

 from these gangliform bodies pass to the grey 

 matter of the convolutions. Some, however, 

 turn inwards towards the mesial plane, and 

 form the corpus callosum by their union with 

 those of the opposite side. 



It cannot be supposed that all the remaining 

 fibres, after subtracting those which form the 

 corpus callosum, pass through the thalami and 

 corpora striata. The disproportion of number 

 between the fibres of the medulla oblongata 

 and these is too striking to allow such an hy- 

 pothesis. They mingle with the vesicular mat- 

 ter of both ; some do not pass beyond them ; 

 others are continued into the medulla oblon- 

 gata, either to its olivary or its pyramidal co- 

 lumns. 



Corpora striata and optic thulaml. The 

 corpora striata and optic thalami bear a strong 

 resemblance in general character and structure 

 to ganglia. They are ovoid masses placed be- 

 tween the fibrous substance of the hemisphere 

 on the one hand, and the medulla oblongata on 

 the other. These bodies, which are best dis- 

 played by laying open the lateral ventricle 

 (p. 675), are very closelv united to each other. 

 The corpus striatum is placed a little in front 



