814 PHYSIOLOGY OF THE ORGANS OF SPECIAL SENSE. 



The homologous stimuli are effective with respect to the sense-organs 

 only within certain limits of intensity. Exceedingly feeble stimuli, to 

 begin with, are without any effect. The degree of intensity of stimu- 

 lation that originates the first trace of sensation is called the threshold of 

 sensation, or the "threshold value." With increase in the intensity of 

 the stimulus the sensations increase, and the sensations increase equally 

 when the intensity of the stimulus increases in like proportions. For 

 example, the same sensation of equal increase in brightness is produced 

 by the light of n candles, instead of 10, or of no candles instead of 

 100 (the ratio of increase in each case being equal to one-tenth). As the 

 logarithms of the numbers increase equally, the law has been expressed 

 as follows : The sensations increase not as the absolute intensities of the 

 stimuli, but approximately as the logarithms of the intensities. The 

 universal applicability of this psychophysical law of Fechner has, 

 however, been disputed recently by E. Hering. Specific stimuli of 

 excessive activity give rise to peculiar painful sensations, as, for instance, 

 the sense of blinding, of deafening of the ear, etc. The sense-organs 

 react to adequate stimuli only within certain definite limits; as, for 

 instance, the ear responds to vibrations of sonorous bodies only with- 

 in the range of a definite number of vibrations, and the retina only 

 to the undulations of the luminiferous ether between red and violet, 

 although not to the heat-waves nor to the chemically active vibrations. 



The designation after-sensation is applied to the phenomenon 

 that the sensation, as a rule, lasts longer than the stimulus; to these 

 belong the after-images, the persistent sensation after pressure upon the 

 skin, etc. Subjective sensations, finally, are brought about by the 

 irritation of the nervous part of the apparatus by internal, somatic causes. 

 The highest order of these subjective sensations, which usually depend 

 upon pathological irritation of the psychosensorial, cortical centers 

 are known as hallucinations ; as, for example, when a person in delirium 

 sees forms or hears voices that are not present. In 'contradistinction to 

 these, the designation illusions is applied to the modifications, by the 

 mind, of a sensation actually present ; as, for example, when the rolling of a 

 wagon is thought to be thunder. Each of these subjects will be taken 

 up in detail under the individual sense-organs. 



In newborn infants the sense of touch is strongly developed, the pain-sense 

 poorly; muscular sensations are doubtfully present; while smell and taste are 

 frequently confused. Auditory stimuli are perceived from the second day on, 

 visual stimuli immediately after birth, but a peripheral visual field does not yet 

 exist. Toward the fourth or fifth week movements of convergence and accom- 

 modation are observed, while after four months colors are differentiated. Dif- 

 ferent stimuli are not perceived simultaneously a reflex inhibitory center is not 

 yet developed. 



