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by the elephant and the whale; and in weight relative to that of the body by many of the smaller birds. There must be farther considered the proportion of the grey to the white tissue.* The four particulars combined, absolute size, size in reference to the body, size of the cerebrum in reference to other ganglia, the proportion between the grey and the white matter, assign man his pre-eminent position. "We shall see the importance of the last particular when we remember that the nervous system is an instrument of transmitting impressions, and also of thoughts: that physical activity and mental activity are dependent, the first more immediately on the medium of transmission, the white matter; the second on the medium of interior activity, the grey matter. Great muscular development, as in the bird, may, therefore, carry with it a relatively large nervous development.

"As we rise through the mammalian series towards man, we find not only a marked increase in the absolute bulk of the cerebral hemispheres, and a yet greater relative excess in their size as compared with the aggregate of that of the sensory ganglia, but an augmentation of their functional powers beyond all proportion to their size, which is derived from the peculiar manner in which their ganglionic matter is disposed. In all ordinary ganglia, the nerve-cells, on whose presence their special attributes depend, form a sort of internal nucleus; but in the cerebrum they are spread out on the surface, forming an external or cortical layer. This layer is covered by a membrane termed the pia mater which is entirely composed of blood-vessels, held together by connective tissue; and thus a copious supply of blood is brought to this important part. But, the extent of the cortical layer, and of its contact with the pia mater is enormously increased by its being thrown into

* Journal of Nervous and Mental Disease, Jan., 1876. First Article.