1068 ON CUTANEOUS AND [BOOK in. 



who had been born blind and restored to sight in adult life, 

 could recognize at first sight and by sight alone a cube, a 

 square, and a sphere. It is 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 

 intellectual efforts and reasoning. A group of visual sensa- 

 tions, constituting a visual image, may have an ordinary objec- 

 tive cause, but may be an ocular illusion ; and the test which 

 we at once apply to determine this is that of touch ; the ordi- 

 nary 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 can- 

 not, 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. 



665. 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, mechani- 

 cal 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 ( 656) 

 in which a heated spoon applied to the skin produced a sensa- 

 tion not of heat but of contact, points perhaps to the affirma- 

 tive, 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 



