9B PHILOSOPHY OF ZOOLOGY. 



c. Claws. These resemble hoofs in structure and situa- 

 tion, deriving their origin from the skin, having a bony 

 centre, and occurring at the extremities of the fingers and 

 toes. 13 ut they arc of more lengthened form, usually ta- 

 pering to a point, convex on the upper surface, and con- 

 cave beneath. Many quadrupeds are furnished with them, 

 and they are usually found on the toes of birds, reptiles, 

 and amuilose animals with articulated members. 



d. Nails. Nails differ from horns and claws, in the cir- 

 cumstance of not being tubular, but consisting of a plate, 

 generally convex on the outer surface, and concave beneath. 

 Where they occur, they occupy the same position as the 

 claws, covering the upper surface of the last joints of the 

 fingers and toes. They derive their colour from the 

 membranes which are beneath. They grow by the root ; 

 so that the parts which are worn off, are soon supplied. 

 They occur on the toes of quadrupeds, birds, and reptiles, 

 and pass, by insensible gradations, into claws. 



c. Spurs. These occur chiefly on what is termed the 

 leg (tarsus) of gallinaceous birds. They are found, like- 

 wise, on the ornithorynchus. Like horns, they are sup- 

 ported in the centre by bone. 



Horns, hoofs, and similar parts, bear a close resemblance 

 to one another in chemical composition. When heated, 

 they soften, and may be easily bent or squeezed into parti- 

 cular shapes. They consist of coagulated albumen, with a 

 little gelatine ; and, when incinerated, yield a little phos- 

 phate of lime. 



Their use, in the animal economy, is to protect the soft 

 parts from being injured by pressure against hard bodies. 

 They are in general wanting, where the parts are in no dan- 

 ger of suffering from the influence of such agents. When 

 torn off from the base, they are seldom completely renewed, 



