728 SPECIAL SOMATIC AND VISCERAL RECEPTORS 



Recurring to the analogy of the house with the many windows, it 

 may then be said that these different openings are beset with a special- 

 ized form of protoplasm which, owing to its chemico-physical constitu- 

 tion, is especially sensitive to energy impressions from without, and is 

 also modified in such a way that it can receive only one particular kind 

 of stimulus. In other words, these sense-organs or receptors show 

 individual differences which render them more particularly adapted to 

 a certain type of energy manifestation. Thus, the retina of our eye 

 cannot be activated by sound waves, nor can the organ of Corti be 

 stimulated by the ethereal impacts of light. Each receptor, therefore, 

 is set aside for only one kind of stimulus and remains impervious 

 to others not specifically suited for it. It is true, however, that in 

 most cases these receptors may be subjected to non-specific stimuli 

 artificially, but the effects are then quite different from normal. 

 Thus, it is possible to stimulate the retina either mechanically or 

 electrically, in which case visual sensations in the form of phosphenes 

 will be obtained, but these sensations are obviously very different from 

 ordinary light impressions. 



This specificity of the different sense-organs is to be attributed to 

 the peculiar structure and composition of the neuroplasm forming 

 them. Their function, however, is materially enhanced in many cases 

 by the fact that the nervous terniinals are amplified by certain acces- 

 sory structures which tend to concentrate the stimuli precisely upon 

 the receptor so as to intensify their force. Thus, we find that the 

 essential receptive element of the eye is the retina, while the different 

 refractive media of this organ, together with the iris, merely serve the 

 purpose of concentrating the light rays upon its most sensitive con- 

 stituent. The same is true of the organ of Corti, because the exter- 

 nal ear, the eardrum, and ossicles merely serve to increase the striking 

 force of the sound waves and to direct them to the sensitive epithelium 

 in the cochlea. 



The orifices in our integument in which the different sense-organs 

 are placed, form, so to speak, points of least resistance through which 

 the energy manifestations in space may reach the interior of our body. 

 But, as has been stated above, each gateway permits of the entrance 

 of only one particular kind of impact, because its resistance toward the 

 others is heightened sufficiently to exclude them. In general, it 

 may be said that we are subject to two types of energies, namely, the 

 vibratory and the chemical, which in turn have been classified by 

 Herrick^ as follows: 



A. Vibratory Energy. 



1. Mechanical impacts received by the tactile corpuscles of the skin at a rate 

 of as high as 1552 vibrations in a second. 



2. Slow vibrations in material media received by the organ of Corti, and sub- 

 jectively perceived as sound. The human ear is activated by vibrations varying 

 between 30 and 30,000 in a second. In some cases, however, this range may be 

 extended to 40,000 and 50,000 vibrations per second. 



1 An introduction to Neurology, Saunders Co., Philadelphia, 1915. 



