APPENDICES OF THE SKIN. 97 



It is permanent, or does not experience those periodical 

 renovations which we have stated to take place with hair and 

 feathers. The deciduous horns of the stag are different in 

 their nature from true horns, and will be afterwards taken 

 notice of. 



The term Horn is usually restricted to the coverings of 

 the projections of the frontal bones of oxen, sheep, and si- 

 milar quadrupeds ; but various appendices of the skin, com- 

 posed of the same materials, and equally permanent, al- 

 though seated on other parts of the body, may, with pro- 

 priety, be included under the same appellation. Among 

 these may be enumerated, 



a. Beaks. The substance which covers the external 

 surface of the maxillary bones of birds, is composed of 

 horn. It varies greatly in texture and markings, according 

 to the species. It differs from the horns of quadrupeds 

 chiefly in this circumstance, that the colouring matter is not 

 so completely fixed, but is probably preserved in vessels 

 which are still in communication with the body. At least 

 it is well known, that the colour of the bill varies with the 

 season, and experiences considerable changes after death. 

 This, however, is most remarkable in those birds, the horny 

 covering of whose bills is ihin, so that the colour may de- 

 pend on the fluids below. 



Horny beaks are found on birds and many reptiles ; and, 

 in the inferior classes, in all those whose jaws are destined 

 to cut hard substances. 



b. Hoofs. Hoofs resemble horns in their manner of 

 growth, and in containing a central support, formed by the 

 termination of the extreme bones of the feet. They grow 

 from the inner surface and base, and are thus fitted to sup- 

 ply the place of those parts which are worn away by being 

 exposed to friction against hard bodies. Hoofs are peculiar 

 to certain herbivorous quadrupeds. 



VOL. I. G 



