PIGMENT CELLS. 



541 



ordinarily move the rotatory, the undulatory, and the 

 waving, like a field of wheat set in motion by a steady 

 breeze. No satisfactory explanation has been given of the 

 cause of this vibratile motion. The current produced by 

 them is from within outwards, in most places ; in the 

 respiratory passages, on the contrary, it is from without 

 inwards. In the Frog's mouth it takes the same course. 

 The ciliary motion may be seen in the kidney of the Frog 

 or Newt ; the cilia in the latter continue in active motion 

 for some minutes after the animal is dead. Make a very 

 thin section of the kidney with a sharp knife, and take 

 care to disturb the structure as little as possible ; then 

 moisten it with a little of the serum of the animal, place it 

 in a glass cell, and cover with thin glass. 



Pigment. Pigment granules are found in greater or 

 less quantities in the skin and bodies of white and dark 

 races. In the eye there is pigment, and it affords a good 

 example of nucleated cells, in which are contained the pig- 

 ment particles, fig. 275. These are placed there for an 

 optical purpose, that of absorbing 

 the rays of light. In the peculiar 

 colouration found in the eyes of some 

 animals, called Tapetum lucidum, the 

 colour is not owing to the pigment 

 particles, but to the interference of 

 the light : it is reflected from it, as in 

 mother-of-pearl, coloured feathers, 

 scales of fishes, &c. The colour of the 

 skin is owing to the granulous con- 

 tents of the pigment cells ; these are 

 like ordinary elementary granules, 

 with the addition of colour; and this latter may be removed 

 by the action of chlorine. 



The Nails are appendages to the epidermis, and present 

 a mould of the cutis beneath ; from the cutis the materials 

 are furnished for the formation and growth of the nail. 

 Like the epidermis, the nail is stratified, the markings 

 are parallel to the surface, and the appearance is produced 

 by the coalescence of the cells and their lying over each 

 other. This arrangement gives an iridescence to sections 

 examined with polarised light. 



Fig. 275. Pigment cells 

 from the eye. 



