AND PHYSIOLOGY. 



207 



Cells, which give color to the skin. These cells are best ex- 

 hibited in the eye where the pigmentum nigrum (black paint) 

 is secreted, and are of the same kind with those in the epi- 

 dermis. They are oval or rounded granules, measuring 

 jtkittfh of an inch in diameter, and one quarter of this in 

 thickness, sometimes presenting a polygonal or stellate form. 

 They have nearly the same composition as the coloring matter 

 of the cuttle-fish, which contains a much larger proportion of 

 carbon than is contained in most organic substances, namely 

 53 parts in hundred. The development of these cells de- 

 pends mainly upon exposure to the sun's light. Hence we 

 see that persons with a fair skin become of a darker hue, if 

 exposed to the strong and direct light of the sun. 



522. The Nails; Mode of Growth; Rate of Growth.— 

 The Nails are composed of the same material as the epidermis, 

 being merely an altered form of it. When their newest por- 

 tions are examined with the microscope, they are found to be 

 nucleated cells closely resembling those of the epidermis, 

 Epithelium cells. The 

 nail increases in length 

 by successive additions 

 to its root, which push 

 it forward over the end 

 of the finger, while at 

 same time it receives 

 additional layers from 

 the skin beneath. The 

 nails of the hands grow 

 about two fifths of a 

 line per week, while 

 those of the foot re- 

 quire four times that period for the same amount of growth. 

 The blood-vessels of the nails are very abundant, and are 



Fig. 289. 



Section of the Thumb, a. Last J>one of the 

 Thumb. 1). Epidermis reflected on the Nail, c 

 Nail. d. Epidermis at the Point of the Tumb. 



What is very remarkable about the chemical composition of this pigment? What 

 does the development of these cells depend on? 522. Of what composition are the nails? 

 Of what kind of cells are they ? How fast do the nails of the hand grow ? 



