236 DR. BEALE^ ON THE TISSUES. 



into certain parts and organs wliich are set apart for the per- 

 formance of distinct offices. The body of a vertebrate animal 

 contains, as we all know, bones, muscles, fat, the liver, kid- 

 neys, the brain, and nerves, &c. 



Each of these may be resolved into elementary organs. An 

 entire bone may be regarded as consisting of an assemblage 

 of certain small portions, each of which contains every struc- 

 ture essential to the constitution of bone, and necessary for 

 its growth. A lung, or a kidney, or the liver, may, in the 

 same manner, be shown to consist of elementary lungs, 

 kidneys, or livers, although these cannot always be perfectly 

 isolated. 



In different animals, the size of these elementary organs 

 differs, but not the same extent as their number. An organ 

 of a large animal, like the whale, differs from the correspond- 

 ing organ of a small one like the mouse, enormously as to the 

 number of elementary organs of which it is made up, but in 

 a much less degree as to the size of each of these. 



Each elementary part is composed of several structures 

 having very different properties. An elementary lung is 

 composed of a delicate, transparent membrane, with elastic 

 tissue, vessels, a prolongation of the bronchial tube. These 

 structures are themselves compound. Connected with the 

 smallest arteries we find nerve-fibres, elastic tissue, muscular 

 tissue, and epithelium. The nerve-fibres, muscular fibre, and 

 epithelium, are composed of elementary parts, and each elemen- 

 tary part consists of matter of two states — germinal matter, 

 active and growing, capable of multiplying itself ; formed ma- 

 terial, passive and incapable of multiplying itself, which was 

 once in the state of germinal matter. An elementary part 

 (cell) of the liver in the same way is composed of the germinal 

 matter within and the formed material externally — the outer 

 part of the formed material is gradually altered, and at last 

 converted into bile and a substance easily converted into 

 sugar. 



An elementary part of bone consists of a mass of germinal 

 matter, external to which is formed material, which gradu- 

 ally becomes impregnated with calcareous salts from without 

 inwards, channels (canaliculi) being left, along which fluids 

 pass to and from the germinal matter, which gradually be- 

 comes inclosed in a space (lacuna). 



An elementary part is seldom more than the 1 -1000th of 

 an inch in diameter, and it may be less than the l-20,000th of 

 an inch. In the adult organism it is often difficult to recog- 

 nise the elementary parts in all cases, in consequence of 

 changes having occurred in the course of their growth, but 



