732 SPECIAL SOMATIC AND VISCERAL RECEPTORS 



all, but are projected to the sense-organ in which they have arisen and 

 even beyond the latter to the area in space from which they have been 

 derived. Thus, the sensations of touch are referred to the surface of 

 the body and those of taste to the tongue and mouth. Quite similarly, 

 the sensations of sight and hearing are always interpreted as having 

 been received from a particular realm in space. It must be evident, 

 therefore, that a sensation cannot be regarded as a definite cortical 

 concept, because since judgment enters this act, which always com- 

 prises other activities, the resultant impression is really the symbolical 

 picture of the conditions mediated from without. 



Each sensation, however, is symbolized independently of the others, 

 because the sensations of light are distinct from those of taste, smell, 

 hearing and touch. This implies that every one of them possesses a 

 definite quality or modality which we have learned to recognize in 

 the course of time. In spite of this fact, however, it would be quite 

 impossible for us to compare these modalities with one another. To 

 be sure, we clearly understand what is meant by the sensations of bit- 

 ter, an intense Hght, a low pitched sound, and other impressions, al- 

 though we are unable to estimate the precise character of these quahties 

 so as to be able to say, for example, that this sound is louder than this 

 taste is sweet. Consequently, each sensation possesses a modality 

 which is absolutely specific to it. Sensations mediated by one and 

 the same sense-organ, however, we are able to rate in a quantitative 

 manner, because we clearly recognize differences in the pitch of the 

 sounds and in the intensity of the light, and are able to tell that the 

 sweet taste of a given substance is more pronounced* than that of 

 another. 



Fatigue and Adaptation. — While the different receptors may be 

 activated by stimuli of a non-specific character, they are usually open 

 to those kinds of impacts only for which they are especially con- 

 structed. It may be said, therefore, that the sense-organs form 

 channels of least resistance through which appropriate stimuli are 

 allowed to pass with the least possible loss of energy to them. Be- 

 ginning with this minimal or threshold value, an increased strength of 

 stimulation always gives rise to a more intense sensation, but an upper 

 limit is finally reached beyond which a further augmentation is impos- 

 sible. This augmentation, however, is not always proportional to the 

 intensity of the stimulus, because an undue strength or duration of the 

 stimulation invariably results in a fatigue which seems to affect pri- 

 marily the recipient centers. It is also evident that the sense-organs 

 are much more receptive toward repeated stimuli of minimal strength 

 than toward single stimuli of maximal strength. In addition, they 

 possess the power of adaptation, which in many cases may be employed 

 to intensify the primary sensation. Thus, we speak of a dark-adapted 

 eye, i.e., an eye which is at first kept in complete darkness so that 

 it may later on be activated by much lower intensities of light than are 

 ordinarily required to stimulate one which has just been exposed to light. 



