PHYSIOLOGY OF THE ORGANS OF 

 SPECIAL SENSE. 



INTRODUCTORY REMARKS. 



The function of the organs of special sense is to transmit to the 

 sensorium impressions of the various phenomena of the outer world; 

 they act, therefore, as the intermediate apparatus of sensory per- 

 ceptions. In order that these may be brought about, the following 

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

 apparatus, must be anatomically intact, and be capable of performing 

 its physiological function. (2) A " specific stimulus" must be present 

 and act upon the end-organ in a normal manner. (3) There must 

 be an uninterrupted communication from the sense-organ through the 

 course of the afferent nerve to the brain. (4) At the time of stimulation, 

 psychic activity (attention) must be directed toward the process of 

 stimulation; in this manner the sensation (as, for instance, of light or 

 sound) first originates through the sense-organ. (5) If, finally, through 

 a psychic act, the sensation is referred to its external cause (a process 

 that'takes place in the cerebral cortex of te psychosensorial centers), 

 a conscious sense-percept is formed. Often, however, this reference 

 is made unconsciously, inasmuch as it is deduced only from experiences 

 previously made. (6) The sensory nerves are connected not only with the 

 cerebral cortex, but also with more deeply situated central nuclei, where- 

 by reflexes are produced, which (in the absence of a conscious sensory 

 perception) appear as movements, for the purpose of guarding the 

 sensory mechanisms against irritation, and protecting them. In the 

 lower animals the instinctive movements for the material preservation 

 of the animal that take place on irritation of the sense-organs are 

 effectuated in this way. 



Among the stimuli that affect the terminal apparatus of the sense- 

 organs there are distinguished: (i) Adequate or homologous stimuli, 

 that is, those for the activity of which the organ is especially constructed, 

 for example the rods and cones of the retina for the undulations of the 

 luminiferous ether. Thus, there is a specific stimulus for each sensory 

 nerve-ending (Johannes Miiller's law of specific energy). (2) Other stimuli 

 of a different nature (mechanical, thermal, chemical, electrical, internal 

 somatic) are also efficient, as, for instance, the seeing of stars in con- 

 sequence of a blow upon the eye, or ringing in the ears as a result of 

 cerebral hyperemia. These heterologous stimuli may affect the nervous 

 elements of the sensory apparatus throughout their entire course from 

 the terminal sense-organ to the cerebral cortex. On the other hand, the 

 adequate stimuli act only upon the terminal apparatus ; for example 

 light thrown upon the trunk of the exposed optic nerve has no effect 

 whatsoever. 



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