RECEPTOR ORGANS 515 



and deep. The last group is derived from receptors in the deeper structures and 

 appears to have given rise to some error in previous experiments on the time of 

 regeneration of sensory fibres. The whole of the deep structures in the hand, for 

 example, are not supplied by the same nerve. The protopathic system is common 

 to the whole body. It regenerates more rapidly than the epicritic system and is 

 of a more primitive nature. The position of the point stimulated cannot be 

 recognised and a widespread, radiating sensation arises from it. The receptors do 

 not appear to be as sensitive as those of the other types but, when excited, the 

 effect is a powerful one. In the skin, they respond to painful stimuli and to 

 extremes of heat and cold. The receptors of deep sensibility answer to pressure and 

 to movement. The fibres from the limbs run in the motor nerves. The system 

 of Pacinian bodies is associated with deep sensibility. The epicritic system is a 

 highly differentiated one and regenerates very slowly. It is only found in the 

 skin and endows it with sensibility to delicate touch, with the power of localisation 

 of stimuli, of distinguishing two points and of discriminating fine degrees of heat 

 and cold, together with the other forms of special sensation. It is obviously the 

 system connected with the development of specialised receptors and the intellectual 

 powers resulting from them. Full details will be found in the paper by Head and 

 Rivers (1908). Page May's article (1909) may also be consulted. 



SMELL 



It is impossible to make a distinction between the differentiated chemical sense 

 of water animals and that of smell in land animals. We are, therefore, justified 

 in regarding the delicate appreciation of the neighbourhood of certain kinds of 

 objects, an endowment which can be shown to be independent of sight and 

 possessed by so many marine animals, as a form of the sense which we know as 

 smell. 



The great enemy of the scallop is the starfish, so that, as soon as the near, presence of a 

 starfish is recognised by the mollusc, its curious swimming movement is executed by this latter 

 and it escapes beyond reach of its enemy. Although the scallop possesses numerous well- 

 developed eyes around the edge of the mantle, stimulation of these by the shadow of the enemy 

 is not sufficient to excite flight, for the reason which we shall see presently. But W. J. Dakin 

 (see von Uexkiill, 1912, p. 329) showed that a small quantity of an extract of starfish, dropped 

 by a pipette in the neighbourhood of a scallop, causes an immediate swimming away. This is 

 a clear instance of the differentiated chemical sense which we may call smell. 



A detailed investigation of the sense of smell in higher vertebrates has been 

 made by Zwaardemaker (1902). It may become of a most extraordinary delicacy, 

 thus : T ^ mg. of mercaptan in. 230 cub. m. of air can be detected, and, of course, 

 only a few cubic centimetres of this air are necessary for the production of 

 the sensation, so that somewhere about 1 x 10~* mg. is sufficient to excite the 

 receptors for smell. 



The paper by Parker (1913, 2). may be consulted with regard to the analogies 



between taste and smell. The latter is the more sensitive. 







PHOTO-RECE PTORS 



Radiant energy, from the sun or similar source, may be absorbed by the skin, 

 converted into heat and thus excite heat receptors. But light waves have also a 

 powerful effect in producing many chemical reactions, and these chemical changes 

 may be such as to stimulate special end organs. Further, as we shall see in more 

 detail in Chapter XIX., which may with advantage be read before the present 

 section, any particular chemical reaction is produced only by a certain group of 

 wave lengths, so that the possibility is presented of distinguishing between light 

 of different wave length, that is, a sensibility to colour. 



It is probable that appreciation of light and darkness by some photo receptor, 

 sensitive to a photo-chemical change in a substance with which it is in contact, 

 would be the first to appear. By this means the proximity of food or enemy 

 would be recognised, although the aid of some other receptors, probably those of 

 a chemical sense, such as smell, would be required to distinguish between the two. 



