Book II. ANIMAL ANATOMY. 285 



the inner surface and base, and are thus fitted to supply the place of those parts which 

 are worn away by being exposed to friction against hard bodies. Hoofs are peculiar to 

 certain herbivorous quadrupeds. 



1864. Claws resemble hoofs in structure and situation, deriving their origin from the 

 skin, having a bony centre, and occurring at the extremities of the fingers and toes. 



1865. Nails differ from horns and claws, in the circumstance of not being tubular, 

 but consisting of a plate generally convex on the outer surface, and concave beneath. 



1866. Spurs occur chiefly on what is termed the leg {tarsus) of gallinaceous birds. 

 They are found, likewise, on the ornithorynchus. Like horns, they are supported in 

 the centre by bone. 



1867. 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 particular shapes. They consist of coagulated albumen, with a little gelatine ; and, 

 when incinerated, yield a little phosphate of lime. 



1868. Tkeir use, in 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 danger of suffering from the influence 

 of such agents. When torn off from the base, they are seldom completely renewed, although very remarkable 

 exertions are frequently made by the system to repair the loss. 



1869. Scales vary remarkably in their form, structure, mode of adhesion, and situation 

 in different animals. In general they are flat plates, variously marked. In some cases 

 each scale consists of several decreasing plates, the lowest of which is largest; so that the 

 upper surface becomes somewhat imbricated. Some scales adhere by the whole of their 

 central surface ; while others resemble the human nail, in having the outer extremity 

 free. 



1870. Shells consist of layers of an earthy salt, with interposed membranes of animal 

 matter, resembling coagulated albumen. They grow by the addition of layers of new 

 matter to the edges and internal surface. When broken, the animal can cement the 

 edges and fill up the crack, or supply the deficiency when a portion is abstracted. 



1871. The earthymatter of shells is lime, in union with carbonic acid. Phosphate of lime has likewise 

 been detected, but in small quantity. The colour is secreted from the animal, along with the matter of 

 the shell. 



1872. Crusts are, in general, more brittle in their texture than shells. They exhibit 

 remarkable differences as to thickness and composition. They differ from shells chiefly 

 in containing a considerable portion of phosphate of lime, and in a greater subdivision of 

 parts. In some cases, however, as the crusts of the bodies of insects, the earthy matter 

 is almost absent, and they may be regarded as formed of cuticle alone. When they contain 

 much earthy matter, as in the crusts of lobsters, the epidermis may be detected as a cover, 

 and the corium beneath may be perceived as a very thin film. In many cases, these 

 crusts are renewed periodically ; and, in all, they are readily repaired. Crusts occur in 

 insects, the Crustacea, and the Echinod^rmata, or sea-urchins, and star-fish. 



1873. These different appendices of the skin pass, by insensible degrees, into one another, as hair into 

 spines, horns into nails, scales into shells, and crusts into membranes. They have all one common origin, 

 namely, the skin j and independently of secondary purposes, they all serve for protection. 



1874. The secretions of the skin are of three kinds ; one class performing the oflice of 

 lubricating the skin, another of regulating the temperature of the body, and a third that 

 of carrying oflP the superfluous carbon. 



1875. Unctuous secretions are confined to animals which have warm blood, and the cells of the cellular 

 web filled with fat. Mammalia and birds. 



1876. Fiscous secretions. In the animals with cold blood, secretions are produced, by the skin, of 

 substances differing in quality from those of warm-blooded animals ; but destined to serve the same 

 purposes, namely, to protect the skin from the action of the surrounding element. 



1877. Sweat, in ordinary cases, exudes from the skin in a state of vapour ; and when 

 condensed consists of water with a small portion of acetic acid and common salt. This 

 secretion is considered as intended to regulate the degree of animal heat, and prevent its 

 accumiilation beyond certain limits. 



1878. Carbon is also emitted by the skin, and appears to be in effect a secondary kind of respiration, but 

 the discovery is but recent (See Ellis on the Germination of Seeds and Respiration of Animals, 1807 and 

 1811.) 



1879. Absorption. There are several circumstances which prove that the skin of the 

 human body, in particular states, is capable of exerting an absorbing power. Whether 

 the absorption takes place by peculiar vessels, or by the exhaling vessels having their 

 motions reversed, or whether absorption ever takes place in the state of health, are 

 questions to which no satisfactory answer has been given. 



Sect. II. Internal Anatomy of Animals. 



1880. Animal anatomy admits of three divisions, the osseous, the muscular, and the 

 nervous structure of animals. 



