16 



THE STRUCTURE OF LIVING THINGS. 



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

 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 whole 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 as protoplasm, and a more solid, 

 rounded body, the nucleus. 



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

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

 which 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 partitions are composed of 

 lifeless matter which has been manufactured and deposited by 

 the living protoplasm constituting the bodies 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 

 lifeless matter enclosing spaces filled 

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



Among animals analogous cases 

 are common. The muscles of the 

 small intestine, for instance, (Fig. 

 5,) consist of bundles of elongated 

 cells (fibres) each of which is com- 

 posed of living matter surrounded 

 by a very thin covering (sheath) of 

 lifeless matter. In cartilage or 



o 



gristle, which covers the 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 01 ' mm g wliat K11OWI1 as the 



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

 cells lying inthe bony life less ma- ^ 



trix. Diagramatic. flattened or irregular cells (cor- 



puscles) are separated by a lifeless 

 fluid (plasma) in which they float. In bone (Fig. 8) the cells 



ill 1 



