16 



THE STRUCTURE OF LIVING THINGS. 



Lifeless Matter in the Living Tissues. In the tissues the Hv- 

 ing cells are seldom in contact one with another, but are more or 

 less completely separated by partitions of lifeless matter. This 

 may be seen in a section through some rapidly growing organ 

 like a young shoot (Fig. 1). The wliole mass is formed of 

 nearly similar, closely crowded units or cells separated by very 

 narrow partitions. Each cell consists of a mass of granular, 

 viscid, living substance known 2,'^ jprotoijlasm^ and a more solid, 

 rounded body, the micleus. 



In such a group of cells no tissues can be distinguished ; or, 

 rather, the whole mass consists of a "single tissue (meristem), 

 wliich is almost entirely composed of living matter (protoplasm). 

 In older tissues the partitions often increase in thickness, as 

 shown in Fig. 2. In every case the ])artitwns are composed of 

 lifeless matter which has heen niannfactitred and deposited hy 

 the living jprotoplasni constituting the hodies of the cells. In 

 still older parts of the plant certain of the lifeless walls may 

 become extremely thick, the protoplasm entirely disappears, and 



the whole tissue (wood) consists of 

 Kf eless matter enclosing spaces filled 

 with air or water (Figs. 3 and 4). 



Among animals analogous cases 

 are common. The nuiscles of the 

 small intestine, for instance, (Fig. 

 5,) consist of bundles of elongated 

 cells {fihres) eacli of which is com- 

 posed of living matter surrounded 

 by a very tliin covering (sheath) of 

 lifeless matter. In cartilao^e or 

 gristle, which covers tlie ends of 

 many bones (Fig. 6), the oval cells 

 are very widely separated by the 

 deposition between them of large 

 quantities of solid lifeless matter 



Fig. 8. (Modified from Schenk.)— Sec- /? • i < • i ^^ 



tion of bone from the human femur lOrmmg what IS kuown aS the 



showing the living branching bone- matrix. In blood (Fig. 7) the 

 cells lying inthe bony life less ma- n ,, i . in/ 



trix. Diagramatic. flattened , or irregular cells {cor- 



jpuscles) are separated by a lifeless 

 fluid {j>lasind) in which they float. In bone (Fig. 8) the cells 



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