730 SPECIAL SOMATIC AND VISCERAL RECEPTORS 



The preceding discussion must have shown that the central nervous 

 system really performs two important functions, namely, to control 

 the activities of the different tissues and organs, and secondly, to bring 

 the latter into a proper relation with the outside world. Since this 

 control is effected with the help of two different groups of nervous 

 structures, the central nervous system may be divided theoretically into 

 a visceral and a somatic part. For the same reason, the sense-organs 

 may be classified as visceral and somatic, the former having to do with 

 the sensations arising within our body, and the latter with those pro- 

 duced by the energies in space. It should be noted, however, that 

 these groups of organs are not functionally isolated from one another, 

 but are closely correlated so that they can always influence one another. 

 Thus, we find that a visceral sensation may give rise to a somatic 

 response, and vice versq. This classification has been made more em- 

 bracing by Sherrington, 1 who divides the somatic group of receptors 

 into exteroceptors and proprioceptors, and the visceral group into 

 general and special interoceptors, as follows : 



A. Somatic receptors, having to do with the orientation of the animal toward its 

 environment. 



1. Exteroceptors, are stimulated under ordinary conditions by outside forces 

 They embrace the end-organs for: (a) touch and pressure, (b) pain, (c) heat and 

 cold, (d) general chemical sensibility, (e) hearing, and (/) vision. 



2. Proprioceptors, are situated in the muscles, tendons and joints and are con- 

 cerned with the production of the muscle-sense. To this group also belong the 

 end-organs which have to do with the sensations of equilibrium, namely, the oto- 

 lithic cavities (static sense) and the semicircular canals (dynamic sense). 



B. Visceral receptors, are concerned with the stimulations arising within the body, 

 principally in connection with digestion, secretion, the action of the heart, and 

 other functions. 



1. General interoceptors, embrace the end-organs, mediating the sensations of 

 hunger, thirst, nausea, respiratory and circulatory sensations, sexual sensations, 

 visceral pain, and others. 



2. Special interoceptors, consist of the end-organs for taste and smell. Both 

 are excited by chemical stimuli and while both are typical interoceptors to begin 

 with, the organ of smell eventually becomes more closely associated with outside 

 conditions. 2 In the amphibians and allied animals it is really the chief extero- 

 ceptor, although its more primitive interoceptive qualities are still in evidence. 



Like the animals, plants are also exposed to varying conditions in the environ- 

 ment and are in possession of intensifying receiving organs. As such may be 

 classified the bristles upon the leaf of Dionaea as well as those upon the stems and 

 leaves of Mimosa pudica. 



The Doctrine of the Specific Nerve Energies. It is by no means 

 difficult to see that the energy manifestations give rise to very specific 

 sensations and reactions. The question then arises, whether this 

 specificity is due to peculiarities in the energy or to peculiarities in the 

 structure of the sense-organs. Johannes von Miiller favors the second 

 view without, however, positively referring it to any particular segment 

 of the neuron. In more recent years physiologists have gone a step 



1 The Intergrative Action of the Nervous System, New York, 1906. 

 * C. J. Herrick, Jour. Comp. Neurol., xciii, 1908, 157. 



