$14 PRINCIPLES OF GENERAL PHYSIOLOGY 



the logarithms of the stimuli are plotted as abscissae, and the sensations as 

 ordinates, a straight line is produced. Victor Henri et Larguier <lcs liuncels 

 (1912) point out that the law applies to vision in the middle of the range of stimuli 

 only. The curve, drawn as above, with logarithms as abscissa;, is, as a whole, 

 of an S-shape. Similar relations between stimulus and effect apply to the 

 electrical changes in the retina, according to the results of De Haas, and also, 

 as regards the middle region, where the curve is practically a straight line, to 

 the action of ultra-violet light on Cyclops, as studied by Mme. Victor Henri et 

 Victor Henri, whose work will be referred to in a future chapter. The conclusion 

 to be drawn is that, in all probability, Weber's law rests on physiological 

 phenomena as its basis. 



THE RECEPTORS IN THE SKIN 



We pass on to refer, somewhat briefly, to certain facts regarding the different 

 kinds of sense receptors. 



Owing to its interest and importance and partly, perhaps, on account of the comparative 

 ease with which it can be investigated up to a certain point, this branch of physiology has 

 produced as much work as any other, probably more. Certain individual sense organs, as the 

 eye and the ear, have large textbooks and memoirs devoted to the^n alone, or even to 

 particular aspects of them, as, for example, the refractive properties of the dioptric system of 

 the eye. I must be content, therefore, with referring the reader to some of these memoirs for 

 the greater part of the information available (see the list of literature at the end of this 

 chapter). 



It is well known that a variety of sensations are obtained by means of the 

 skin. Heat and cold have been already referred to ; these were shown by Blix 

 (1884) to arise from distinct spots, as also the sense of pressure. Von Frey 

 (1894) showed that there are also distinct pain spots. To the latter investigator 

 we owe most of our knowledge on the question, and his article (1913) may be 

 read with profit. Von Frey's use of delicate hairs, the degree of pressure exerted 

 by each being determined by the weight required to bend it, should be mentioned. 



It is impossible to give, at present, an adequate explanation how small 

 differences of heat and cold are magnified sufficiently to excite nerve fibres. 

 Various possibilities might be mentioned, such as a chemical reaction greatly 

 accelerated by heat or some physical mechanism making use of expansion to cause 

 pressure. Pressure stimuli themselves may perhaps be increased by some kind 

 of lever action, as in the case of the long bristles forming the whiskers of the cat, 

 which seem to be sensitive even to air currents, since it is difficult otherwise to 

 suggest an explanation of their guiding power in the dark. 



The various modifications of the sense of touch are brought about by combina- 

 tion of movement with contact, by which a series of sensitive points is excited, 

 or the same spot by a series of stimuli occurring at different rates. 



In this latter connection, the possibility may be referred to that a nerve fibre may have 

 synapses with two neurones and that the refractory period in one synapse may be longer than 

 in the other. . Thus, a slow rate of stimuli may pass both unaltered, while a rapid rate will 

 pass only one unaltered, being reduced to a slow rate in the other one. In this way, there 

 seems to be an indication of the way one nerve fibre might serve to convey impulses giving 

 rise to different sensations. An economy of nerve fibres might possibly be brought about, a 

 point of importance in connection with the very fine gradations in the higher senses, but it is 

 purely hypothetical. There is another point to be remembered here. The roughness of a 

 surface appears to be the same although the finger is moved over it at different rates. This 

 fact indicates that it is not merely the different number of stimuli affecting the nerve ending 

 in a given time that conditions the nature of the sensation, but that this is combined with the 

 sensation of movement given by the muscular sense. So that doubt is cast on any interpreta- 

 tion which assumes that the absolute number of stimuli per second is a controlling factor in 

 different species of sensations. 



Protopathic and Epicritic Sensibility. This is the appropriate place to refer to 

 the experiments of Head, Rivers, and Sherren (1905) on the time of regeneration 

 of various skin sensations. Head caused to be divided the radial nerve (a purely 

 sensory nerve) in his own arm and observations were then made on the returning 

 sensibility. The results led him to regard the sensations derived from the appli- 

 cation of stimuli to the hand to be of three different groups : protopathic, epicritic, 



