Physiology of the Sense Organs. 



383. INTRODUCTORY OBSERVATIONS. Requisite Conditions. The 

 sense organs have the function of transferring to the sensorium impressions of the 

 various phenomena of the external world ; they are, in fact, the intermediate 

 instruments of sensory 'perceptions. In order that this may occur, the following 

 conditions must be fulfilled : (1) The sense organ, provided with its specific end- 

 organ, must be anatomically perfect, and capable of acting physiologically. (2) 

 A " specific stimulus " must be present, which under normal conditions acts upon 

 the end-organ. (3) The sense organ must be connected with the cerebrum by 

 means of a nerve, and the conduction through this path must be uninterrupted. 

 (4) During the act of stimulation, the psychical activity (attention) must be 

 directed to the process, and then the sensation results, e.g., of light or sound, 

 through the sense organ. (5) Lastly, when, by a psychical act, the sensation is 

 referred to the external cause, then there is a conscious sensory perception. Often, 

 however, this relation is completed as an unconscious conclusion, as it is essentially 

 a deduction from previous experience. 



Stimuli. With regard to the stimuli which are applied to the sensory apparatus, we dis- 

 tinguish : (1) Adequate or homologous stimuli, i.e., stimuli for whose action the sense organs 

 are specially adapted, such as the rods and cones of the retina for the vibrations of the ether. 

 Thus, each sense organ has a specific form of stimulus best adapted to act upon it. This is 

 what Johannes Midler called the "law of specific energy." (2) There are many other forms 

 of stimuli (mechanical, thermal, chemical, electrical, internal somatic) which act upon the sense 

 organs, producing the flash of light beheld when the eye is struck ; singing in the ears' when 

 there is congestion of the head. These heterologous stimuli act upon the nervous elements of 

 the sensory apparatus along their entire course, from the end-organ to the cortex cerebri. The 

 homologous stimuli, on the other hand, act only on the end-organ, i.e., light has no effect 

 whatever upon the trunk of the exposed optic nerve. 



Strength and Liminal Intensity. Homologous stimuli act upon the sensory 

 organs only within certain limits as to strength. Very feeble stimuli at first 

 produce no effect. That strength of stimulus which is just sufficient to cause the 

 first trace of a sensation is called by Fechner the " liminal intensity " of the 

 sensation. As the strength of the stimulus increases, so also do the sensations, but 

 the sensations increase equally when the strength of the stimulus increases in 

 relative proportions. Thus, we have the same sensation of equal increase of light 

 when, instead of 10 candles, 11, or instead of 100 candles, 110 are lighted the 

 proportion of increase in both cases is equal to one-tenth. As the logarithm of the 

 numbers increases in an equal degree, when the numbers increase in the same relative 

 proportion, the law may be expressed thus : " The sensations do not increase with 

 the absolute strength of the stimuli, but nearly as the logarithm of the strength of 

 the stimulus." This is Fechner's "psycho-physical law," but its accuracy has 

 recently been challenged by E. Hering. [It holds good only with regard to stimuli 



