SKELETON. 373 



And is composed of : — 



Animal substance entirely : or 



Animal substance and calcareous earth : or 



Calcareous earth entirely. 



"When of animal substance entirely, it may be — 

 Membranous, as in 



Many parts of young animals ; some parts of the skeleton 

 of some fishes : 



Gelatinous, as in 



Soft-shelled sea animals [Tunicata] : 



Cartilaginous, as in 

 Some fishes : 

 Many parts of the more perfect animals : 



Or horny, as in 



Flying insects. 



The hardest or firmest parts of an animal, whose use in the machine 

 is to give support and form to the whole, and attachment to the moving 

 powers, may be called the ' skeleton :' or if only to a part, then it is 

 the skeleton of that part. Some consist of only one piece 1 , others are 

 made up of several pieces, having motion on each other, which at the 

 same time directs and determines the places for motion. 



A skeleton respecting the internal economy of the animal is of no 

 use, having no action within itself so as to influence others ; it is pas- 

 sive, as a wheel in a machine, and must be acted upon. However, its 

 use is not so essential as that of a wheel ; for wheels make a part of 

 the internal economy of the machine. But wheels neither give sup- 

 port nor shape, which are two very essential uses of the skeleton. 



Skeletons are composed of different kinds of substances suited to the 

 necessity ; some being extremely hard, others soft, yet of sufficient 

 firmness to give support, and afford fixed points for muscular attach- 

 ments. These substances may be membrane, cartilage, horn, and 

 bone. The first three are animal substances, the last is a kind of mix- 

 ture of animal matter and earth. These different substances are not 

 entirely peculiar to particular classes of animals ; some having two 

 kinds, others three, and probably there may be some which have all the 

 four. We find, too, that some animals may have one kind at one period 

 of life, while they have another kind at another period. Some skele- 

 tons contain the whole animal, as in the oyster ; others are different in 



[Hunt. Preps. Phys. Series, Nos. 225, 226.] 



