Page:Popular Science Monthly Volume 84.djvu/131

Rh been derived from one of the olfactory type by a proximal migration of its cell body. The stimulation of its free-nerve terminals may be conceived to take place, as Botezat has recently pointed out, through the secretory activity of the surrounding epithelial cells as a result of their contact with the stimulating solution, rather than from the direct action of this solution on the nerve endings themselves. From this standpoint the epithelium comes to be an essential element in the stimulation of the neurone and affords, so to speak, a favorable sensory environment for the real nerve-endings. Finally, the gustatory neurones may be said to have appropriated certain of these epithelial cells which have become differentiated into taste buds and whose activity, probably secretory in character, to follow Botezat, is called forth by the superimposed solution and is essential to the stimulation of the nerve endings. Thus in the evolution of the chemical sense organs of vertebrates certain integumentary cells originally quite independent of the receptors came to be involved with these and were eventually appropriated by them as essential parts of the gustatory apparatus. This process of appropriation is not unlike that seen among the effectors and represents one of the important steps by which the nervous system in the course of its evolution has added to its complexity. Although the nervous system probably arose in a scattered way at spots where the primitive multicellular animal had developed muscle, it became unified through the need for general transmission tracts, and, by increasing its own elements as well as by appropriating additional effectors and receptors, it has impressed upon the higher animals, including ourselves, a unity so profound that it includes everything that we mean by personality.