304 ON CUTANEOUS AND [BOOK in. 



perhaps especially in relation to size and space, that the two senses 

 work together. 



There are no converse cases of persons who, born without touch, 

 and trusting to sight alone have, in later life, had touch restored 

 to them ; but there are many things within our vision, which are 

 beyond our touch at the moment and some which we can never 

 touch at any time ; our conceptions of these latter are more or 

 less uncertain, and the direct visual sensations have to be 

 strengthened or corrected not by mere sensations but by intel- 

 lectual efforts and reasoning. A group of visual sensations, con- 

 stituting a visual image, may have an ordinary objective cause, 

 but may be an ocular illusion ; and the test which we at once 

 apply to determine this is that of touch ; the ordinary idea of a 

 ' ghost ' is that of a something which we can see but cannot touch, 

 which excites visual sensations but affords no tactile sensations. 

 Conversely a touch by something invisible, a touch as of a body 

 which we ought to be able to see but cannot, we also recognize as 

 unreal. The concordance of touch and vision affords in fact to 

 a large extent the standard by which we judge of the reality of 

 things. 



897. The last remark naturally leads to the statement that 

 as in the case of the other sensations, so in the case of the several 

 cutaneous sensations, we may have sensations which are not due 

 to their ordinary objective causes. 



We have seen that visual sensations may arise from changes 

 in the retina started not by light but by other agents, mechanical 

 and others ; and the question presents itself, Can touch proper, the 

 sensation of pressure, be excited otherwise than by pressure and 

 sensations of temperature by changes in the skin other than those 

 of temperature ? No very definite answer can be given to this 

 question, though the case quoted above ( 888) in which a heated 

 spoon applied to the skin produced a sensation not of heat but of 

 contact, points perhaps to the affirmative, as does also the fact 

 that electric currents applied to the skin may produce sensations, 

 pricking sensations, which if not identical with, may at least be 

 confused with those of pressure. 



Cutaneous sensations of all kinds may however be of central 

 origin, may be due to changes in the central nervous system quite 

 independent of all events in the skin, and may yet be referred to 

 this or that region of the skin and to the objective cause which 

 ordinarily gives rise to the sensation. Painful sensations indeed 

 may rise from changes not only in the central organs but at any 

 part of the whole length of the nerve, all being referred to the 

 cutaneous terminations of the nerves on which the cause of pain 

 is usually brought to bear. Tactile and temperature sensations 

 as we have said cannot originate in changes in the nerves them- 

 selves, but they may arise through changes in the central organs ; 

 we may be subject to tactile phantoms comparable to ocular 



