NEItVOUS CENTRES. (HUMAN ANATOMY. THE EMCEPIIALON.) 



Fig. 394. 



695 



Superior surface of the right hemisphere of the adult human brain. 



The undulating form of many of the convolutions is very well seen, and the general characters of the 



convoluted surface are displayed. 



of the rabbit, the beaver, the guinea-pig, the 

 agouti shew these fissures. They are generally 

 regular in different individuals of the same 

 genus, and they are symmetrical, i. e., of the 

 same length and direction, and occupy the 

 same place on each hemisphere. 



Leuret remarks, in reference to the dogma 

 of Gall and Spurzheim, that the presence and 

 number of the convolutions are in direct rela- 

 tion to the volume of the brain, that such is far 

 from being universally the case ; and I am 

 glad to refer to so excellent an authority in 

 confirmation of the view which I have advo- 

 cated respecting the true signification of the 

 cerebral convolutions. According to this ana- 

 tomist, the ferret, which has several well-marked 

 convolutions on each hemisphere, has a brain 

 no larger than that of the squirrel, which is 

 entirely devoid of them, and which has not 

 even the few fissures which faintly indicate 

 their first developement in the brains of the 

 rabbit, the beaver, the agouti, &c. And these 

 animals last named have the brain actually 

 larger than that of the cat, the pole-cat, the 

 roussette, (Pteropusvulgaris,) the unau, ( Bra- 

 dypus didactylus,) the sloth, (Bradi/pus tridac- 

 tylits, ) and the pangolin, all of which possess 

 convolutions. 



All mammiferous animals, excepting those 

 mentioned in the preceding paragraphs, have 

 convolutions which exhibit more or less of 

 complication. This complication has evidently 

 no connection with the general organization o'f 

 the animal, inasmuch as we find animals, in 

 the same family with those which possess 

 numerous convolutions, exhibiting a very slight 

 developement of them. The monkeys, the 

 dolphin, the elephant, exhibit the most" nume- 

 rous convolutions of any of the Mammalia 

 inferior to man, in whose brain the convoluted 

 surface reaches its highest point. 



Each fold on the surface of the brain is 

 ordinarily called a convolution, whatever be 

 its position, size, or direction. It consists of 

 a fold of grey matter, enclosing a process of 



white or fibrous matter. On each side of it is 

 a sulcus or groove, in which we find the same 

 elements, a fold of grey or vesicular matter 

 concave externally, convex internally the 

 fibrous matter adhering to its convex surface. 

 As the convolution exhibits no essential diffe- 

 rence of structure from the sulcus, it is plain 

 that the former portion of the brain's surface 

 cannot differ in physiological office from the 

 latter. We describe particular convolutions, 

 not because they are to be regarded as 

 endowed with special functions distinct from 

 the less prominent portions of the cerebral 

 surface, the sulci, which are continuous and 

 identical in structure with them, but because 

 they afford good indications of a particular 

 arrangement of the surface of the hemispheres 

 by which one brain may be coveniently com- 

 pared with another, whether they belong to the 

 same or to different groups of animals. 



The folded arrangement of the surface of 

 the hemispheres, dependent as it is upon the 

 grey matter, is evidently destined to bring the 

 central and deep-seated parts of the hemispheres 

 into union with a large extent of vesicular 

 surface. 



That the disposition of the convolutions, 

 like that of all other parts of animal bodies, 

 follows a particular law, is well illustrated by 

 comparing the brains of different groups of 

 animals, in their gradation from the more simple 

 to the more complex. 



M. Leuret very justly makes a distinction 

 between those convolutions which are constant, 

 and to be found throughout the whole series of 

 convoluted brains, occupying the same position, 

 and differing only in size and extent of connec- 

 tions, and those which are not constant, even 

 in the brains of the same group of animals, 

 but are dependent on the extent of the primary 

 ones, and the connections which they form 

 with others near them. According to this idea 

 we may classify the convolutions as primary 

 ami secondary, 



The primary convolutions are all formed after 



