14 THE STRUCTURE OF LIVING THINGS. 



ofanorganism. The cell is an ^ orgamc individual of the first 



uVtag td liLess Matter in the Living Organism. Since our 

 own bols and those of lower animals and of p ants are coin- 

 Ted of matter, it may be supposed, from what has been said 

 in the last chapter, that they are composed of living 

 matter This, however, is true only in part. 

 strictly true that every plant or animal contains living 

 matter but a little reflection will show that it contains 

 lifeless matter also. In the human body lifeless mat- 

 ter is found in the hairs, the ends of the nails, and 

 the outer layers of the skin, structures which are 

 not simply devoid of feeling, as every one knows them 

 to be but are really lifeless in every sense, although 

 formino- part of a living body. Nor is lifeless mat- 

 ter confined to the exterior of the body. The mineral 

 matter of the bones is not alive; and this is true, 

 though less obviously, of many other parts, such as 

 the liquid basis or plasma of the blood, the fat (which 

 is never wholly absent), and various other forms of mat- 

 ter occurring in many parts of the body. 



In lower animals examples of this truth occur on 

 every hand. The calcareous shells of animals like the 

 snail and the oyster ; the skeletons of 

 corals and sponges ; the hard outer crust 

 of insects, lobsters, and related animals ; 

 the scales of fish and reptiles; the 

 feathers, claws, and beaks of birds ; the 

 fur of animals these are a few of the 

 countless instances of structures com- 

 posed wholly or in part of lifeless mat- 

 FIG.S. (AfterRanvier.)-Mus- ter ^^^ nevertheless enter into the 



cle-cells. A, from the intes- 4 . , 



tine of a dog, in cross-sec- composition oi living animals. 



won; B, single isolated ceil, Among plants like facts are even 



from the intestine of a rab- & ^ r 



bit, viewed from the side, more conspicuous. No one can doubt 

 that the outer bark of an oak is devoid 



of life. The heart-wood of a tree is entirely dead, and even 

 in the so-called live wood, through which the sap flows, not only 

 is the solid part of the wood lifeless, but also the sap itself. 



