RECEPTOR ORGANS 511 



the irritant action of the substances named. This view is confirmed by the fact that twice 

 molar sugar, which cannot be supposed to be nocuous, has no effect. 



The senses of taste and smell are higher developments of this primitive 

 chemical sense. Delicate special receptor organs have been formed, especially 

 in the land vertebrates. That of smell has, indeed, become a kind of distance 

 receptor, of great importance in many animals, owing to the stimulant substance 

 being conveyed by air or water. It might be thought that the name of smell 

 should be confined to the appreciation of vapours, but it must be remembered that, 

 before acting on receptors, these vapours must enter into solution in the liquid 

 covering the receptors. There is, therefore, no sufficient reason to make a 

 distinction between this sense in the shark and in the dog. It is obvious, 

 however, that transmission through the air by currents is more rapid than 

 through water, and that, for this reason, the growth of the mechanism as a 

 distance receptor, becomes more obvious in land animals. The nature of smell 

 as a distance receptor in fishes is discussed- by Parker and Sheldon (1913). 



The sense of taste is much less delicate than that of smell, and cannot be said 

 to play a great part in the growth of the higher nervous systems. It has scarcely 

 any distance element, even in its most developed form. 



It is remarkable that, even in the higher vertebrates, the sensory neurones of smell receptors 

 have retained their primitive condition of cell body in the epithelium itself with nerve processes 

 passing into the central ganglion. But it seems doubtful whether this fact altogether justifies 

 the view suggested by Parker (1912) that the sense of smell represents the ancestral chemical 

 sense. The epithelial cells would very early form chemical products by the action of external 

 chemical agent^, products of such a nature as to stimulate the free nerve endings between the 

 epithelial cells and thus form a common basis front which the more delicate mechanisms of 

 smell and taste have been developed. At the same, time, we must be careful in limiting these 

 activities to purely chemical ones, since it is difficult to imagine any chemical property common 

 to lead acetate, saccharin and glucose. The acid taste, apparently, is merely a question of 

 hydrogen ion concentration, and it is not difficult to suppose the existence of some peripheral 

 chemical substance very sensitive to this factor. 



TOUCH RECEPTORS 



In addition to the action of chemical substances in the water surrounding 

 them, primitive organisms are exposed to contact with other objects. Such 

 contacts probably, at first, acted as nocuous stimuli merely, of a nature in- 

 distinguishable from one another, but evoking the powerful nociceptive reflexes. 



It is curious that, according to Cohnheim (1912, 2, p. 112), the molluscs known as " Hetero- 

 pods " are insensitive to the presence of food until it comes into contact with them, apparently 

 being unable to appreciate it by chemical sense or by vision, although they possess eyes. After 

 biting an object, however, they appear to recognise by taste whether it is fit for food or not. 



From the varied effects produced by substances in contact with the skin, the 

 elaborate system of skin receptors, as we know it in ourselves, has been 

 differentiated. But before we proceed further to the consideration of the higher 

 sense organs, and especially of the distance receptors, a few additional introductory 

 remarks of a general nature are necessary. 



THE RECEPTOR MECHANISM IN GENERAL 



Organisms, as remarked above, are the better provided for their adaptation to 

 changes in their environment, the greater the number and variety of external 

 phenomena which they are capable of appreciating. To be aware of things happen- 

 ing at a distance gives opportunity for preparation to meet them before they actually 

 arrive. Hence the great advance in animal organisation with the development 

 of organs, such as the eye and the ear, which enable things at great distances to 

 impress themselves. 



It is also very necessary that many events occurring in the organism itself 

 should be made known to the nerve centres. We have, as well as extero-ceptors, 

 intero-ceptors, as they are called by Sherrington, and, amongst these, the , proprio- 

 ceptors are of great importance. These are the receptors in an active organ which 

 give information to the centres of the state of activity of this organ, and are thus 



