696 



NERVOUS SYSTEM. (NERVOUS CENTRES. THE ENCEPHALON.) 



one type. Of this, as M. Leuret suggests, the 

 brain of the fox may be taken as the basis. 

 The fissure of Sylvius is well marked in this 

 brain ; it is bounded by a prominent convolu- 

 tion, which encloses it above, below, and 

 behind thus forming a curve, the concavity of 

 which is directed forwards and downwards. 

 Above and behind this we find a second con- 

 volution forming a similar curve and parallel to 

 the first. It exhibits a slight undulation, and 

 is marked by a short fissure signs of ad- 

 vancing complication. Still further back and 

 upwards there is a third convolution, parallel 

 and curved similarly to the second ; this bifur- 

 cates at one point. Above all, near the 

 summit of the hemisphere, a fourth is found 

 disposed in the same curved manner, but ex- 

 hibiting some sinuosities or undulations at its 

 anterior portion. A fifth convolution exists on 

 the inferior surface of the anterior lobe and 

 rests upon the roof of the orbit. Leuret de- 

 signates it the supra-orbitar convolution. The 

 sixth convolution is of great extent ; the prin- 

 cipal portion of it is found on the inner surface 

 of each hemisphere above the corpus callosum; 

 in front it bends downwards and backwards to 

 the fissure of Sylvius, and behind it extends to 

 the middle lobe and forms the hippocampus 

 major. This convolution exists in a high slate 

 of developement in the human brain, and has 

 attracted very generally the attention of ana- 

 tomists. Foville describes it by the name 

 convolution d'ourlet. 



Such is the most simple arrangement of the 

 convolutions. The complication of this takes 

 place by undulations being formed in the con- 

 volutions themselves, by a subdivision of them 

 at certain situations, by the junction of neigh- 

 bouring ones through smaller folds crossing the 

 siilci between them, and in the highest classes 

 by the addition of totally new convolutions. 



Animals, whose brains have nearly the same 

 degree of developement as that of the fox, 

 have exactly the same convolutions, differing, 

 hosvever, somewhat in point of size. This in- 

 crease of size is denoted by undulations formed 

 in the course of convolutions throughout more 

 or less of their extent. The dog may be taken 

 as an example. M. Leuret states that, in com- 

 paring the brains of several dogs together, he 

 found with all of them the same convolutions, 

 differing only in the extent of undulations and 

 the number of depressions, both of which were 

 greatest in the largest brains. The brain of a 

 large mastiff (chien dogue), a good watch-dog, 

 of such great ferocity that he attacked the person 

 who fed him, had all the convolutions very 

 large and much undulated, with numerous 

 depressions in them. 



A group of animals, consisting of the cats 

 and the hyena, exhibits another stage of in- 

 crease in the developement of convolutions. 

 The same type prevails as in the fox and dog ; 

 four external convolutions, one internal, and a 

 supra-orbitar. These convolutions, however, 

 are united to each other at numerous points by 

 means of small folds crossing the sulci. These 

 uniting folds form the secondary or supple- 

 mentary convolutions. Nearly all the primary 



convolutions have supplementary ones con- 

 nected with them. 



A group, which includes the sheep and other 

 ruminant animals, exhibits much more com- 

 plication in the cerebral convolutions, but still 

 preserves the same type. The undulations 

 and the supplementary convolutions are more 

 numerous. The primary appear less nu- 

 merous because less distinct. The anterior 

 part of the internal convolution is much in- 

 creased in developement, and the supra-orbitar 

 is much more complex. In the fissure of 

 Sylvius some small convolutions are found 

 which are the first developement of those which 

 in the human subject constitute the insula of 

 Reil. 



In the brain of the elephant new convolu- 

 tions are added. These consist of folds pass- 

 ing in a perpendicular direction ; the primitive 

 convolutions always taking a longitudinal 

 course. These latter are divided by the former 

 into an anterior and a posterior set. Others 

 are found above and in front of the fissure of 

 Sylvius; three superior convolutions are found, 

 the continuations of which backwards are 

 situate above the internal convolution. All 

 the convolutions of the elephant are remark- 

 ably undulating and exhibit numerous depres- 

 sions. The brain of the whale is very similar 

 to it in this respect, and both resemble that of 

 man. 



Monkeys have not the tortuous or com- 

 plicated convolutions which are found in the 

 whale and elephant. Yet the developement 

 of the hemispheres at their posterior part, the 

 general form of the brain, the extent and in- 

 clination of the fissure of Sylvius approximate 

 the brain of monkeys to that of man much 

 more nearly than the whale's or elephant's, 

 which, notwithstanding their complicated con- 

 volutions, are generally inferior in organization, 

 and resemble the brains of other Mammalia. 

 The internal convolution in monkeys is simple; 

 below and behind it forms the hippocampus, 

 from which convolutions are prolonged back- 

 wards, forming the posterior lobe. Two supe- 

 rior convolutions are met with above the fissure 

 of Sylvius, between which is placed a trans- 

 verse fissure very constant, called the fissure of 

 Rolando. The orbitar convolutions are largely 

 developed. 



In comparing the human brain with that of 

 the inferior animals, we notice great exactness 

 of symmetry between the convolutions of op- 

 posite hemispheres in the latter, and the want 

 of it in the former. It cannot, however, be said 

 that the convolutions of opposite hemispheres in 

 the human subject are absolutely unsymmetri- 

 cal. A careful examination will show that the 

 same convolutions exist on each side, but appa- 

 rently of different sizes, and not closely correspon- 

 ding as regards situation. My meaning will be 

 more readily understood by referring to fig. 

 381, p. 671, where the same numbers have 

 been affixed to corresponding convolutions. 

 No. 1 on the right has a certain general re- 

 semblance with No. 1 on the left, which would 

 be much more perfect but for the fissure 

 which marks the convolution of the right he- 



