RECEPTOR ORGANS 513 



touch in the skin nerves, and so on. It is obvious that the differences recognised 

 by the organism must be due to "analysers" in the central nervous system. 

 Something has already been said on this point in discussing Pavlov's conditioned 

 reflexes (pages 503-504), and attention may be called to Fig. 158 (page 504). We 

 meet with the phenomenon again in the form of Mliller's law of "specific 

 energies," as he called it (1843, p. 1065). The word "energy" is unfortunate 

 here, since it is used in the sense merely of " endowment " or " property " and 

 Mliller himself uses it interchangeably with " quality." The law amounts to this, 

 that, however excited, each nerve of special sense gives rise to its own peculiar 

 sensation. Miiller also (1826, 1840, and 1843) points out that the same external 

 cause may excite a different sensation in different sense organs. " Sensation 

 is not a conduction of a condition of certain external bodies directly 

 to consciousness, but the conduction of a state of a particular nerve, and the 

 excitation of each nerve of special sense has its own peculiar sensation, which 

 cannot be replaced by that of any other nerve." 



The section in Miiller's book (1843, pp. 1059-1087), dealing with the senses in general, may 

 be read with profit. It is probable that some of the misunderstanding of the meaning of 

 Miiller's law is due to the fact that he sometimes speaks of "nerves" when we should now say 

 "nerve centres." The statement on page 1072, however, that "it is certain that the central 

 portions of the nerves, included in the encephalon, are susceptible of their peculiar sensations, 

 independently of the more peripheral portions of the nervous cords, which form the means of 

 communication with the external organs of sense," shows that he was fully aware of the fact 

 that the nerve trunks act as conductors only. 



The fact that sensation of light is evoked by section of the optic nerve is often 

 brought as proof of the statement before us, but, since a slight pull on the retina 

 very easily excites the receptors there, it is, by itself, only evidence that the 

 mechanical stimulation of the retina, as well as that by light, can produce the 

 special sensation. The best proof is that afforded by the trunk of the chorda 

 tympani nerve as it passes through the tympanic cavity, as referred to above 

 (page 388). A sensation of taste is produced by either mechanical, chemical, or 

 electrical stimulation at this point. 



Why these nerve impulses, alike in themselves, give rise to such widely different sensations 

 when they arrive at their respective cerebral terminations is a matter beyond the scope of 

 physiological analysis. We have to make use of the fact, to a certain extent, in the investiga- 

 tion of the laws of action of the peripheral receptors. 



The next point to be noted is that, however these receptors themselves are 

 excited, whether the stimulus is one for which they are specially adapted or not, 

 the sensation evoked is the same. Thus, there are certain spots in the skin which 

 give rise to a sensation of heat, even when excited electrically or mechanically, 

 while there are others which give rise to a sensation of cold when similarly excited. 

 The important point is that specialised receptors are much more sensitive to their 

 appropriate stimulus than to any other. An electrical stimulus strong enough to 

 excite a heat spot excites also a cold spot or a pressure spot, whereas a warm body is 

 able to excite a heat spot when its stimulus is too weak to excite any other kind 

 of receptor. A warm surface thus stimulates only the heat spots, until its 

 temperature is raised sufficiently high to be nocuous and to cause pain. It is 

 possible that the inappropriate stimulus needs to be strong enough to excite the 

 actual nerve fibres themselves, in which case we are clearly dealing with Miiller's 

 law itself. 



Weber's Law. As this law is of general application, it may properly be 

 referred to here. It refers to the increase of the sensation, as related to that 

 of the stimulus, and states that, in order to produce a just detectable difference in 

 sensation, an equal fraction of the stimulus must be added to it, whatever its 

 value. Suppose that we could just detect the difference between 10 g. and 11 g., 

 it would be necessary to add 100 g. to a kilogram before the change was noticed. 

 Thus to increase a weak sensation by a given amount requires a less addition 

 to the stimulus than to increase a strong sensation by the same amount. The 

 law is a particular aspect of a law frequently met with in natural phenomena. 



It may be put in another form. To excite a series of sensations differing 

 by equal increments, the stimuli must increase in geometric proportion. If 



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