8 PHYSIOLOGY CHAP. 



have sought to answer this question, but no reply is possible from 

 the standpoint of experimental science. This is one of the 

 transcendental problems to which Du Bois - Reymond replies 

 ignoramus et ignorabimus. But the same answer had already 

 been given by his master Johannes Mliller. " The nature of this 

 state of the nerves whereby they see light, hear sound, the nature 

 of sound as a property of the auditory nerve, of light as a property 

 of the optic nerve, of taste, smell and touch, remain eternally 

 unknown like the final causes in natural philosophy." The 

 modern philosophical principle of the relativity of all knowledge 

 acquired through the senses is a direct consequence of Miiller's 

 law, that our sensations depend upon the innate qualities of our 

 senses, and do not reproduce the phenomena of the outer world. 



(c?) " We do not know whether the different energies of the 

 sensory nerves are intrinsic in them or in the parts of the brain 

 and cord to which they run, but it is certain that the central 

 portions of the corresponding sensory paths within the brain are 

 capable of exciting the corresponding sensations, independently 

 of the nerve-conductors." This conclusion leaves the question 

 undecided whether the specific energies of the senses depend 

 upon a property inherent in the respective sensory nerves or 

 upon their central terminal organ. As we have already seen 

 (iii. p. 262), this question is still unsolved, though the theory 

 Johannes Miiller himself preferred receives most support, viz. 

 the identity of nervous function, on which the nerves are regarded 

 merely as indifferent conductors to the centres of the excitations 

 that arise in the peripheral organs. The specific excitability 

 of the several senses to given stimuli is due to the differentiation 

 of the protoplasm, which is in relation with the nerve-endings of 

 the peripheral sense-organ ; the specifically distinct sensations 

 that arise in consciousness during excitation are due to the 

 dissimilar nature of the central organs ; the sensory nerves that 

 unite the peripheral organs with the central sense-organs are 

 uniform conductors which are identical both in their internal 

 structure and in their function. Hering, nevertheless, maintains 

 the contrary hypothesis, and extends the concept of specific 

 energy not only to the central cells but also to their processes, 

 i.e. to the whole neurone. 



It is very difficult to determine the limits of the law of the 

 specific energy of sensory nerves. The question is whether not 

 modality only, but also the qualitative differences that occur 

 within one and the same modality of sensation, depend on specific 

 energies of the neurones that build up the sensory organ, or 

 whether they can be explained on the assumption that the 

 individual fibres of a sensory nerve are capable of serving different 

 forms of excitation or activity. This question will be discussed in 

 relation to each of the several senses. 



