Page:Popular Science Monthly Volume 71.djvu/115

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109
AGE, GROWTH AND DEATH

over into secretion by a chemical change, it is discharged from the cell, the cell loses in volume and in its shrunken state presents a very different appearance, as is shown at B in the figure. It is necessary for the cells to again elaborate the material for secretion before they can a second time become functionally active. Here we have something of the secret of the production of the various juices in the body revealed to us. Other excellent examples of the differentiated condition of the cells are afforded us by the examination of hairs, of which I will show you two pictures. The first represents a section through the human

PSM V71 D115 Cross section of human skin displaying hair follicles.png

Fig. 13. Section of the Human Skin, made so that the Hairs are cut Lengthwise.

skin taken in such a way that the hairs are themselves cut lengthwise and you can see not only that each hair consists of various parts, but also that the cells in these parts are unlike. The follicles within the skin in which the hair is lodged likewise have walls with cells of various sorts. It may interest you also to point out in the figure the little muscle which runs from each hair to the overlying skin, so disposed that when the muscle contracts the "particular hair will stand up on