Elementary anatomy and physiology : for colleges, academies, and other schools . elementaryanato00hitc Year: 1869 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 larg
Elementary anatomy and physiology : for colleges, academies, and other schools . elementaryanato00hitc Year: 1869 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 ?
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