Herrick, Subdivision of the Brain. 395 



modalities of sense primarily represented here are: (i) cutaneous 

 sensation (touch, temperature, etc.) ; (2) hearing; (3) vision. These 

 are termed the somatic, as distinguished from the visceral, senses, 

 their most essential characteristic being the fact that the reaction 

 evoked is somatic, i. e., a movement of the body as a whole (in 

 lower animals usually locomotor in type) with reference to the 

 source of the stimulus. 



In certain cases, as relatively recent ph^logenetic adaptations, 

 sense organs of the visceral type have secondarily appeared in the 

 skin and assumed exteroceptive functions, as in the cutaneous 

 taste buds, of certain teleosts. But these exceptions must be recog- 

 nized as such and need not confuse our primary subdivisions. The 

 organ of smell presents a somewhat similar atypical instance, 

 which I have considered in more detail in another article. ^ 



2. Proprioceptors. — These systems were evolved parallel with 

 the exteroceptors and subsidiary to them. Their sense organs lie 

 chiefly in the organs of somatic response, i. e., in the muscles, 

 joints, tendons, etc., and are adapted to assist in the correlation of 

 the movements of the body, reporting to the central nervous system 

 the exact degree of contraction of every muscle, the degree of ten- 

 sion on the joints, tendons, etc. They are therefore indispensable 

 to all delicate muscular adjustments involving accurate regulation 

 of movement, strain, etc. Their organs of response are obviously 

 identical with those of the exteroceptors with whose functions they 

 are associated. The most highly specialized member of the group 

 is the mechanism of equilibration in the auditory labyrinth. 



3. Interoceptors. — ^These comprise the visceral systems of sense 

 organs chiefly concerned with stimuli received in the digestive 

 tract, and exciting visceral responses thereto. The most typical 

 sense modalities are taste and a group of ill-defined pneumo-gastric 

 sensations (sufi^ocation, nausea, hunger, thirst, etc.). Smell and 

 taste (the chemical senses) are clearly related physiologically and 

 probably have a common origin from a primitive undifferentiated 

 chemical sense. 



The purpose of the difTerentiation in vertebrates of two chemical senses so closely similar physio- 

 ogically and psychologically and so very dissimilar anatomically has given morphologists a world of 

 trouble. Sherrington suggests the most natural explanation when he calls attention to the fact that, 

 while the organ of taste is a typical interoceptor, the olfactory organ is a distance receptor. In other 

 words, the chemical sense has differentiated along two lines determined simply by whether the source of 



1 On the phylogenetic differentiation of the organs of smell and taste. Journ. Comp. Neurol, and 

 Psych., vol. 18, no. 2. 1908. 



