690 



THE POPULAR SCIENCE MONTHLY. 



with hair, claws, or finger-nails, and is only modified skin. When 

 a section through a piece of skin is examined with the aid of a mod- 

 erately-powerful microscope, the lower or internal surface is seen to 

 be made up of little, irregularly-rounded cells, or bags, with soft semi- 

 fluid contents ; and, while the animal is alive, new cells are constantly 

 forming under the old ones, which are pushed outward and crowded 

 together, and gradually lose their soft contents, and are flattened out 

 into very small scales. The outer layer of the skin is made up of 



Fig. 3. 



Fig. 4. 



Transverse Section op Shaft op Pri- 

 mary Wing-feather op a Goose, magni- 

 fied to show the cellular structure. 



Longitudinal Section op Same, more 

 magnified. 



these scales, which are fastened to each other firmly enough to be 

 separated from the living layer below in a thin sheet, as happens when 

 a blister is raised on the hand by unusual work, but in most parts of 

 the body they are slowly rubbed off as new ones grow; but at the tips 

 of the fingers they are so firmly united that they form horny plates, 

 or " nails," which are pushed forward as new cells form at the root. 



In the skin of a bird where a new feather is to grow there is a 

 little pit, and, at the bottom of this, an elevation or pyramid ; extend- 

 ing up one side of this pyramid is a groove, or furrow, deepest at the 

 base, and gradually growing shallower until it disappears near the 

 top ; from each side of this furrow a great many smaller grooves ex- 

 tend around to the other side of the pyramid, and these also decrease 

 in depth, and at last disappear just as they are about to meet on the 

 side opposite the large furrow. The whole pyramid is covered with 

 skin, and the surface is made of the same scales, or flattened cells, that 

 are found over the rest of the surface of the body ; but, instead of 

 falling off when they are pushed out by the new ones below them, 

 they become united or welded to each other, so as to form a horny 

 coat over the surface of the pyramid, with ridges on its lower or inner 

 surface, corresponding to the grooves on the pyramid ; and, as new 

 cells grow at the base, this coat or cast of the surface is pushed up- 

 ward till it breaks at its thinnest part, which is, of course, the smooth 

 part without ridges opposite the large furrow; and then, as it is 



