284 SCIENCE OF AGRICULTURE. Part II. 



1833. Shells consist of layers of an earthly 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. 



1834. The earthy matter of shells is lime, in union with carbonic acid. Phosphate of lime has likewise 

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

 shell. 



1835. Crusts are, in general, more brittle in their texture than shell. 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 subdivison 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 echinodermata, or sea-urchins, and star-fish. 



1836. These different appendices of the skin Y>ass, 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 ; and independent of secondary purposes, they all serve for protection. 



1837. The secretions of the skin are of three kinds, one class performing the office of 

 lubricating the skin ; another of regulating the temperature of the body ; and a third of 

 carrying off the superfluous carbon. 



1838. Unctuous secretions are confined to animals which have warm blood, and the cells of the cellular 

 web filled with fat, mammalia and birds. 



1839. Viscous secretions. In the animals with cold blood, secretions are produced, by the skin, of sub- 

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

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



1840. Sweat, in ordinary cases, exudes from the skin in a state of vapor; 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 pre- 

 vent its accumulation beyond certain limits. 



1841. 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.) 



1842. Absorption. There are several circumstances which prove, that the skin of 

 tlie 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. 



1843. Animal anatomy admits of three divisions, the osseous^ the muscular, and the 

 nervous structure of animals, 



SuBSECT. 1. Osseous Structure of Animals. 



1844. The organs of external anatomy are generally considered as destined for pro- 

 tection ; while those of the interior of the animal, or the bones, give stability to the 

 power, support the muscles, and afford levers for the execution of locomotion. Bones 

 may be considered in regard to their composition, articulations, and arrangement. All 

 bones are composed of the periosteum, cartilaginous basis, earthy matter and fat. 



1845. The periosteum bears the same relation to the bone as the skin to the body, 

 serving as a covering for its surface, and a sheath for the different cavities which enter it. 

 It varies in thickness, according to the nature of the bone. Its texture is obviously 

 fibrous; and it possesses blood vessels. Its sensibility indicates the existence of the 

 nerves. 



1846. The cartilaginous basis consists of gelatine and coagulated albumen. The 

 earthy matter is chiefly phosphate of lime, and the fat resembles that of the fixed oils. 



1 847. Jiones increase in size, not as in shells, scales, or horns, by the addition of 

 layers to the internal surface, but by the expansion of the cartilaginous basis ; which, 

 when it becomes saturated with earthy matter, is incapable of farther enlargement. This 

 is the reason why the bones of young animals are soft and flexible, while those of old 

 animals are hard and brittle. 



1848. The proportion between the cartilaginous basis, and the earthy matter differs, 

 not only in every animal according to age, the earthy matter being smallest in youth, 

 but, likewise, according to the nature of the bone itself, and the purposes which it is 

 destined to serve. The teeth contain the largest portion of earthy matter. Remarkable 

 differences are likewise observable, according to the class or species. 



