STRUCTURE OF BIRDS. 6t 



marrow, and in those which run rather than fly, some of them 

 are impervious to air. 



I have now only some observations to offer respecting the 

 dermal system, or skin and feathers, after which it may be 

 useful to consider the exterior of the bird, and mention the 

 principal terms applied in describing it. 



The Dermal or Tegumentary System is composed of the 

 dermis, the rete mucosum, and the epidermis or cuticle. The 

 skin, thus constituted, is connected with the subjacent parts 

 by loose cellular tissue, which often contains a great quantity 

 of fat, and is moved by the cutaneous muscles, by which the 

 feathers are elevated and depressed. The dermis is generally 

 very thin ; and the rete mucosum, although usually colourless 

 in the parts that are covered Avith feathers, often exhibits on 

 those that are exposed the most beautiful and varied tints. The 

 epidermis is generally scaly or scurfy on the feathered parts of 

 the body ; but on the tarsi and toes assumes a smooth and 

 polished appearance, and to form the claws and mandibles be- 

 comes thickened, and acquires a horny texture. 



Birds, like quadrupeds, are invested with a covering, which 

 is connected with the skin, and lies immediately upon it. This 

 covering is chemically of the same nature as the hair of mam- 

 mifera. and the scales of reptiles and fishes, but it differs essen- 

 tially in respect to its mechanical structure, being much more 

 complex in its constituent parts than the envelope of these 

 classes of animals. To this general envelope the name of 

 plumage is given. In ordinary language it is more frequently 

 called the feathers. 



It may be presumed that the plumage of birds serves to pro- 

 tect them from the injurious agency of external powers, such 

 as cold, heat, rain, and hail, and that it operates in retaining 

 the heat generated in the body, and in developing or fostering 

 electricity. The varieties of structure, magnitude, and propor- 

 tion, and the degrees of connection, which its parts present, 

 together with the diversified hues, and the varied capabilities 

 of absorbing or reflecting light which it possesses, must, in a sys- 

 tem where everything is the result of design, originate from 

 peculiar specific necessities, and be subservient to the welfare, 



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