PIGMENT CELLS. 261 



ternal surface of the choroid, and also on the cervical portion 

 of the pia mater. 



Mixed up with perfect pigment cells a greater or less 

 number of pigment granules are always observed ; these are 

 amongst the smallest objects in nature, and, on account of 

 their minuteness, it is in them that molecular action in all its 

 activity is best seen : they are not spherical in form, but are 

 flattened, so that they appear as discs, lines, or points, ac- 

 cording as the surface, side, or end presents itself to the eye 

 of the observer. 



It is probable that it is by means of these granules that 

 pigment cells are multiplied. 



Climate, and particular states of the system, as pregnancy, 

 have much effect in increasing the amount of pigmentary 

 matter beneath the skin ; from the latter cause the areolae 

 around the nipples frequently become of a deep chocolate 

 colour. Of the influence of the former it is scarcely neces- 

 sary to cite examples : it may be remarked, however, that 

 freckles are due to the development of pigment cells, brought 

 about by the action of the summer's sun. 



This augmented development of pigment cells may be 

 rationally explained by the increased determination of blood 

 to the dermis of the breast from increased activity of function 

 in that organ during pregnancy, and to the general surface 

 from the effect of the sun's heat. 



It is questionable whether all the varieties of colour of the 

 human species have not originated in climateric causes, ope- 

 rating through many ages. 



It may also be questioned whether pigment cells are not 

 susceptible of being developed into those of the epidermis. 

 If the epidermis of a negro, raised from the surface by means 

 of a vesicatory, be examined with the microscope, it will be 

 noticed that the most external cells contain a considerable 

 amount of colouring matter, which, as this resides in solid 

 granules and not in a fluid, it is difficult to suppose had en- 

 tered the cells by endosmosis. 



Pigment cells are capable of regeneration in a part in 

 which they have been destroyed : it is necessary, however, 



