450 COMPARATIVE HISTORICAL. 



and epidermis. In some reptiles, the epidermis becomes horny, and forms large plates or scales. 

 Similar structures occur in the edentata among mammals. The epidermal appendages assume 

 various forms such as hair, nail, spines, bristles, leathers, claws, hoof, horns, spurs, &c. The 

 scales of some fishes are partly osseous structures. Many glands occur iu the skin ; in some 

 amphibia they secrete mucus, in others the secretion is poisonous. Snakes and tortoises are 

 devoid of cutaneous glands; in lizards the "leg-glands" extend from the anus to the bend of 

 the knee. In the crocodile, the glands open under the margins of the cutaneo-osseous scales. 

 In birds, the cutaneous glands are absent; the "coccygeal glands" form an oily secretion for 

 lubricating the feathers. [This is denied by 0. Liebreich, as he finds no cholesterin-fats in 

 their secretion.] The civet glands, at the anus of the civet cat, the preputial glands of the musk 

 deer, the glands of the hare, and the pedal glands of ruminants, are really greatly developed 

 sebaceous glands. In some invertebrata, the skin, consisting of epidermis and chorium, is 

 intimately united with the subjacent muscles, forming a musculocutaneous tube for the body 

 of the animal. The cephalopoda have chromatophores in their skin, i.e., round or irregular 

 spaces filled with coloured granules. Muscular fibres are arranged radially around these spaces, 

 so that when these muscles contract the coloured surface is increased. The change of colour in 

 these animals is due to the play or contraction of these muscles (Briicke). Special glands are 

 concerned in the production of the shell of the snail. The annulosa are covered with a chitin- 

 ous investment, which is continued for a certain distance along the digestive tract and the 

 trache;e. It is thrown off when the animal sheds its covering. It not only protects the animal, 

 but it forms a structure for the attachment of muscles. In echinodermata, the cutaneous 

 covering contains calcareous masses ; in the holothurians, the calcareous structures assume the 

 form of calcareous spicules. 



Historical. Hippocrates (born 460 B.C.) and Theophrastus (born 371 B.C.) distinguished the 

 perspiration from the sweat ; and, according to the latter, the secretion of sweat stands in a 

 certain antagonistic relation to the urinary secretion and to the water in the faeces. According 

 to Cassius Felix (97 a.d.), a person placed in a bath absorbs water through the skin ; Sanctorius 

 (1614) measured the amount of sweat given off ; Alberti (1581) was acquainted with the hair- 

 bulb; Donatus (1588) described hair becoming grey suddenly; Riolan (1626) showed that the 

 colour of the skin of the negro was due to the epidermis. 



