i 2 6 TEE POPULAR SCIENCE MONTHLY 



shown to be in its action essentially non-nervous. Such highly dif- 

 ferentiated, but independent effectors have, I believe, been appropri- 

 ated 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 ef- 

 fectors, 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 appropria- 

 tions. 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 ex- 

 tend into the central olfactory apparatus. They reproduce in a most 

 striking way the type of primary sensory neurone common to the in- 

 vertebrates, 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 epi- 

 thelium of the moist parts of the vertebrate skin. The gustatory 

 neurones reproduce almost exactly the condition of those of the com- 

 mon 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 undoubt- 

 edly 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 prob- 



