Page:Popular Science Monthly Volume 84.djvu/130

126 shown to be in its action essentially non-nervous. Such highly differentiated, but independent effectors have, I believe, been appropriated from time to time by the nervous system in that during ontogeny certain motor fibers, instead of becoming attached to their appropriate muscles, have wandered to new effectors which have been sufficiently responsive to their stimuli to give a basis for a permanent attachment. Thus the nervous system, once established around muscles, has widened its influence in that it has appropriated other types of independent effectors, which upon application were found to be responsive to its stimulus.

But the differentiated nervous system has not only extended itself on the side of its effectors, it has probably also made receptor appropriations. This is well illustrated by several groups of related sense organs such as the organs of touch and hearing in the vertebrates or those of the chemical senses in the same animals. The latter may serve as an example.

The chemical sense organs in vertebrates include not only those of smell and of taste, but also the organs of the common chemical sense such as are concerned with the chemical irritability of the skin of the frog or of the exposed or semi-exposed mucous surfaces of man. All these chemical receptors are stimulated by solutions. In taste the stimuli are the dissolved materials in the food; in smell they are the solutions formed on the moist olfactory surface from the materials wafted in the air to the nose.

The neurones concerned with the reception of these stimuli exhibit interesting relations. The olfactory neurones, as is well known, have their cell bodies in the olfactory epithelium, whence their neurites extend into the central olfactory apparatus. They reproduce in a most striking way the type of primary sensory neurone common to the invertebrates, and in this respect they represent the most primitive type of sensory neurone in the body of vertebrates. The neurones concerned with the common chemical sense are like those of the olfactory sense except that their cell bodies have migrated centrally and constitute a part of one of the cerebro-spinal ganglia. As a result the distal ends of these neurones are represented as free-nerve terminations in the epithelium of the moist parts of the vertebrate skin. The gustatory neurones reproduce almost exactly the condition of those of the common chemical sense, except that their distal free terminations are around taste buds instead of being in an ordinary epithelium.

The conditions shown by these three types of receptor mechanisms suggest at once a genetic connection. The olfactory type is undoubtedly the most primitive, and stimulation in this instance is initiated by the chemical action of the superimposed solution on the hairs of the olfactory cells. The neurone for the common chemical sense has