RECEPTOR ORGANS 531 



retinal receptors is very small indeed, compared to that of the same form of 

 energy required to excite the heat receptors of the skin, for example. 



In the skin, there are receptors for heat, cold, touch, and pain. These are 

 again grouped by the first relay of central analysers into the two groups of 

 protopathic and epicritic sensibility. These two groups also apply to other regions 

 of the body, some regions, however, being possessed of receptors for the protopathic 

 group only. Their more precise definition will be found in the text. 



Receptors for light are, in all probability, arranged to make use of a photo- 

 chemically sensitive substance. The products of this reaction, or possibly the 

 changes of energy involved in the course of the reaction, are such as to excite 

 the nerve terminations. Thus we may have a primitive sensibility to light 

 situated in the skin generally. 



But, to be of value as a distance receptor, an organ for light stimuli requires 

 to be able to form images of external objects. So that we find a dioptric mechanism 

 present to produce an image on a sensitive surface composed of a number of 

 elements each connected with a separate nerve fibre. 



The different methods of focussing this dioptric mechanism, accommodation for 

 near or distant objects, are given in the text. 



The only known photo-chemical substance present in the retina is the visual 

 purple. It is sensitive to nearly the whole of the visible spectrum, but whether 

 it consists of one substance only, or of several, or whether other photo-chemical 

 substances are present is not yet known. In order to account for colour vision, 

 the photo-chemical changes produced by light of one wave length must differ 

 from those produced by another wave length, so that different receptors may be 

 stimulated. 



That visual purple is, at least, one of the photo-chemically active substances 

 of the retina, is shown by the fact that the light absorbed by it in different 

 parts of the spectrum, the threshold stimulus necessary to produce sensation in 

 the peripheral parts of the retina, and the bleaching effect of the light on the 

 pigment, all follow the same curve. Other properties of the visual purple are 

 described in the text. 



There are certain characteristic electrical changes produced by light acting on 

 the retina. The actual curve obtained experimentally is more or less complex, 

 but can be analysed into a compound of three or more simple curves, each of which 

 has an opposite direction at incidence and at disappearance of light. But these 

 components have no connection with the three hypothetical sensations of the 

 Young-Helmholtz theory. - Since the electrical change in the Cephalopod is less 

 complex in nature, and the nerve elements in this case are separated from the eye 

 itself, it appears likely that some of the complexity of the electrical change in the 

 vertebrate eye is due to these nerve elements. 



There is reason to believe that there are either six or seven primary colour 

 sensations in man, as described by Newton. Further, the whole spectrum does 

 not consist of an indefinite number of gradations of visible tint, but of a seines of 

 patches, each of which, when isolated, appears to be of a uniform colour 

 (monochromatic). The number of these areas is about sixteen to twenty in 

 normal sighted people, but the precise number differs according to circum- 

 stances. 



The mechanism of the receptors for sound consists of a membrane, stretched 

 so as to be in a state of tension transversely only. Its transverse measurement 

 increases regularly from one end to the other, so that it is capable of resonance 

 to different rates of sound vibrations at different regions. The vibrations of the 

 different regions are intensified by the structures known as Corti's organ, in order 

 to be able to excite the nerve fibres of each region. Degenerations can be 

 produced in localised areas by exposure to a particular note for a long time. 



The receptors for sound appear to have arisen somewhat late in the course of 



