8 ZOOLOGY. 



the essential basis of all the solid parts animated by the vital 

 movement. These plastic materials are less varied than might 

 be at first supposed ; for in all animals the basis (trame) of 

 the living parts appears to be composed of a substance called 

 albumen, or of fibrin which probably is but albumen slightly 

 modified. The solids also resemble each other in having 

 water as a constituent part, to the presence of which they, 

 no doubt, owe their flexibility, softness, and other physical 

 properties essential for the due performance of their func- 

 tions. But the mode of texture of the solids thus constituted 

 varies much, and the name of organic tissues has been 

 given to those parts which in their turn reunite to form the 

 organs. 



19. The principal organic tissues of animals are four : 

 the muscular, nervous, cellular, and utricular. 



The muscular tissue forms what is commonly called the 

 flesli ; it is the producing agent of all motion, and is com- 

 posed of fibres, susceptible of contracting or of being short- 

 ened. These fibres, wherever placed, may always be distin- 

 guished by their contractile faculty, and are always found 

 where motion is performed. 



The nervous tissue is soft, and generally whitish ; it forms 

 the brain and nerves ; it is the seat of sensation. 



The connective or cellular tissue, also named areolar or 

 spongy, is, of all the constituent materials of the body, the 

 most abundant. In some of the more simple animals it seems 

 to form the whole body ; in those more highly organized it 

 connects and yet insulates all the organs, entering largely into 

 their composition, and being modified in a variety of ways, it 

 forms membranes and a number of other tissues ; in its sub- 

 stance the fat is always deposited. It is a whitish substance, 

 elastic, semi-transparent, composed of filaments variously 

 interlaced, and of small lamella?, more- or less consistent, and 

 irregularly united, so as to leave between them cells or 

 lacunae of variable size. But the walls of these cells are 

 incomplete, and thus permit fluids (or air) to pass freely from 

 one to another ; these cells are moistened with a watery and 

 slightly albuminous liquid, called serosity. 



The utricular tissue is composed of little cells or bladders, 

 with distinct walls, glued to each other, either directly or by 

 means of an amorphous organic matter ; sometimes these 

 vesicles are rounded, and filled with some particular substance, 

 as fat, for example ; at other times they are found flattened 



