STRUCTURE OF LIVING MATTER 



15 



The first is that Hving bodies always contain the substance 

 known as protoplasm ; in fact most of the processes characteristic 

 of life go on only in the protoplasmic parts, the others being by 

 comparison Hfeless. Protoplasm will be considered in more detail 

 later, but it may be said here that it is an aqueous solution in 

 which protein is the most important constituent. The other parts 

 of the body are formed material, made by the protoplasm ; 

 an example is the ground substance of bone (Fig. 403), consisting 

 largely of salts of Hme, to which it owes its hardness. 



.^.s. 



ABC 



Fig. 6. — Portions of animal tissues, highly magnified, to show cells. 



A, The lining of an artery ; B, muscular tissue from the wall of the intestine ; C, the lining of the intestine. 



A and B are shown in surface view, C in section. 

 c, Cells ; g.s., ground or intercellular substance, traversed by threads of protoplasm from cell to cell. 



The second structural feature of living matter is that it possesses 

 a considerable degree of organisation. In many animals, as we 

 shall see in a later chapter, the protoplasm is not continuous, 

 but is arranged in a number of minute units known as cells 

 (Fig. 6). In each cell a small protoplasmic body, the nucleus, 

 acts as a regulative centre, and on the surface the protoplasm is 

 modified to form a cell wall. If the protoplasm is not divided 

 into cells, nuclei are still present. 



ORGANS 



Apart from the microscopic division of its protoplasm, the 

 living body consists of a number of parts each of which does a 



