90 THE COLOURS OF ANIMALS 



relaxed, so that the skin will be dark ; and we have 

 seen that such a result actually occurs. When a 

 muscle is cut off from nervous stimuli it also enters 

 a condition of permanent rest or relaxation. Thus 

 when the face is paralysed on one side, the muscles 

 are relaxed and unable to balance the contraction of 

 those on the other, so that the face is drawn over 

 towards this latter side. The contractions of a muscle 

 cell and those which take place in a pigment cell are 

 not essentially different ; the former are far more 

 specialised and powerful, but both of them exhibit 

 manifestations of that contractile power which is 

 possessed by the simplest cells. It is therefore of 

 interest that both should behave in a similar manner 

 when cut off from the nervous system which provides 

 the stimuli under which both normally contract. In 

 1858, Sir Joseph Lister showed that the coloured 

 part of a pigment cell contracts independently of the 

 cell itself. Cells are now recognised as composed of 

 a network containing a glassy substance in its meshes ; 

 pigment granules are only contained in the network, 

 and, as this contracts, it carries them inwards from 

 the long branching processes towards the centre of 

 the ceU. 



Loss of colour in cave-dwelling animals 



On the other hand it has been argued that the 

 Proteus, a blind amphibian living in the underground 

 rivers of Carniola, and Carinthia, is light-coloured, 



