PIGMENT-CELLS. 



139 



of an inch in diameter, and a quarter as much in thickness ; these, when 

 separately viewed, are observed to be transparent, not black and opaque ; and 



[Fig. 32. 



A. Choroid Epithelium, with the cells filled with 

 pigment, except at a, where the nuclei are visi- 

 ble. The irregularity of the pigment-cells is seen. 

 6. Grains of pigment. 



B. Pigment-cells from the substance of the Cho- 

 roid. A detached nucleus is seen. Magnified 

 320 diameters.] 



Cells from Pigmentum Ni- 

 grum; a, pigmentary gra- 

 nules concealing the nu- 

 cleus; b. the nucleus distinct. 

 Magnified 410 diameters. 



they exhibit an active movement when set free from the cell, and even whilst 

 inclosed within it. The Pigment-cells are not always of a simple rounded or 

 polygonal form; they sometimes present remarkable stellate prolongations, 

 such as those seen in the skin of the Frog (Fig. 88) ; and occasionally, the 

 cells being more nearly approximated to each other, these prolongations com- 

 municate, so as to form a kind of network. The Chemical nature of the Black 

 pigment has not yet been distinctly ascertained ; it has been shown, however, 

 to have a very close relation with that of the Cuttle-fish ink, or Sepia, which 

 derives its colour from the pigment-cells of the ink-bag; and to include a 

 larger proportion of carbon than most other organic substances, every 100 

 parts containing 58 of that element. 



164. It cannot be doubted that the development of the Pigment-cells of 

 the skin is very much influenced by exposure to light ; and in this respect 

 there is a remarkable correspondence between Animals and Plants, the 

 coloration of the latter, as is well known, being entirely due to that agent. 

 Thus, it is a matter of familiar experience, that the influence of light upon 

 the skin of many individuals, causes it to become spotted with brown freckles ; 

 these freckles being aggregations of brown pigment-cells, which either owe 

 their development to the stimulus of light, or are enabled by its agency to 

 perform a decided chemical transformation, which they could not otherwise 

 effect. In like manner, the swarthy hue, which many Europeans acquire 

 beneath exposure to the sun in tropical climates, is due to a development of 

 dark pigment-cells, and to this we usually find the greatest disposition in in- 

 dividuals or races, that are already of a somewhat dark complexion. The 

 deep blackness of the Negro skin seems dependent upon nothing else than a 

 similar cause, operating through successive generations ( 80). It is well 

 known that the new-born infants of the negro and other dark races, do not ex- 

 hibit nearly the same depth of colour in their skins, as that which they present 

 after the lapse of a few days, when light has had time to exert its influence 

 upon their surface ; and further, that in those individuals who keep them- 

 selves during life most secluded from its influence, we observe the lightest 

 hue of the epidermis. Thus among the intertropical nations, the families of 

 Chiefs, which are not exposed to the sun in the same degree with the com- 



