6 PHYSIOLOGY CHAP. 



This in mi way invalidates the distinction between modality 

 and quality of sensation; it merely emphasises the fact that in 

 the higher sensrs the differentiation in the modality of the 

 sensations is far more pronounced and striking than in the less 

 developed sensrs. 



The different modalities of the sensations do not depend on 

 differences in the external stimuli which excite them, but on the 

 specific nature of the different senses. Johannes Mliller (1840) 

 published a masterly development of this theory, and brought 

 out its full importance alike in physiology and psychology. It 

 is usually known as the "Law of specific energy of the senxcx" 

 (vol. iii. p. 262), and was summed up by M tiller in the following 

 general propositions : 



(a) "No kind of sensation can be produced by external causes 

 which cannot be equally excited in the absenee <>l external causes 

 by intrinsic changes in our nerves." 



Purely internal causes may give rise to sensations of cold, 

 heat, pain, pleasure, which are normally evoked by external 

 stimuli acting on the skin. Certain olfactory and gustatory 

 sensations are termed subjective, because they arise in the absence 

 of any substance capable of arousing smell or taste. Auditory 

 sensations may be due to internal or external causes : buzzing 

 and subjective noises in the ear are common at the beginning 

 of feverish disorders. Visual sensations light, darkness, and 

 colours may occur without extrinsic causes. When the excita- 

 bility of the optic nerve is exaggerated, subjective sensations of 

 light and colour arise even with the eyes shut and in total 

 darkness. Independently of transmission of any stimulus from 

 the peripheral organs the nerve-centres may be thrown into 

 activity by direct internal excitation. Under physiological 

 conditions this happens in rf/v////.s, under pathological conditions 

 in hallucination*. The outer world can therefore make no 

 impression on us which purely internal causes are unable to arouse. 



(&) "The same internal or external cause evokes different 

 sensations through the different senses, according to their nature 

 or their specific sensibility." 



Hyperaemia or congestion of the sense-organs is an internal 

 cause which produces specific effects on the different senses, as 

 buzzing in the ear, flashes of light in the eye, pain in the sensory 

 nerves of the skin or viscera, etc. The electrical current is a 

 classical means of showing that the same external cause may 

 produce sensations of dissimilar modality when it acts on 

 different senses. If applied to the eye the galvanic current 

 evokes luminous sensations, to the nose smell, to the tongue 

 taste, to the skin sensations of pressure, warmth, cold, or pain, 

 according to the nerve-organs encountered at the different parts 

 to which it is directed. 



