To the nature of the convolutions our especial attention must be directed. The brain-substance consists of gray and white nervous matter. The gray matter forms the outermost layer of the brain substance, and incloses the white; the opposite arrangement being seen, curiously enough, in the spinal cord. Now, one evident purpose of the convolutions of the brain is to largely increase the amount of its gray matter relatively to the space in which the organ of mind is contained; while the perfect nutrition of the brain is also thus provided for through its convoluted structure permitting a fuller distribution of the minute blood-vessels which supply the brain with the vital fluid. It is a very noteworthy fact that the structure of the gray matter differs materially from that of the white. In the gray matter nerve-cells are found in addition to nervous fibers, the former originating nervous force, while the latter are simply capable of conveying this subtile force. Thus it may be said that it is in the gray matter that thought is chiefly evolved, and from this layer that purposive actions spring. The white matter, on the other hand, merely conveys nerve-force and nervous impressions, and is thus physiologically inferior in its nature to the gray substance. The observations of Gratiolet, Marshall, and Wagner seem to leave no room for doubt that the convolutions of the brain increase with culture, and are therefore more numerous and deeper in civilized than in savage races of men. It is curious, however, to observe that certain groups of quadrupeds are normally "smooth-brained," and possess few or no convolutions. Such are rats, mice, and the rodents or "gnawing" animals at large, and it can hardly be maintained that in those animals intelligence is normally low or instinct primitive—although, indeed, the just comparison of human with lower instincts must be founded on a broader basis than is presented by this single anatomical fact.
A final observation concerning the anatomy of the brain relates to its size and weight as connected with the intelligence. The phrenological doctrine of the disposition of the faculties must be held to include the idea that the larger the brain, the better specialized should be the mental qualities of the individual; the greater the amount of brain-substance forming the good and bad qualities and regions of the phrenologist, the more active should be the mental organization. Now, it is a patent fact that this rule tells strongly against the phrenologist's assumption. True, various great men have had large brains; but cases of great men possessing small brains are equally common, as also are instances where insanity and idiocy were associated with brains of large size. The normal average human male brain weighs from 49 to 50 ounces; man's brain being ten per cent, heavier than that of woman. Cuvier's brain weighed 6412 ounces; that of Dr. Abercrombie 63 ounces; that of Spurzheim, of phrenological fame, 55 ounces; Professor Goodsir's brain attained a weight of 5712 ounces; Sir J. Y. Simpson's weighed 54 ounces; that of Agassiz 53·4 ounces; and that of Dr.
