156 THE NATUKE AND ORIGIN OF LIFE 



Lung 



Digestive 

 ''Tube 



- Kidney 



FIG. 8. 



For higher animals like man, a remark has to be made 

 with relation to the digestive tube which traverses the body 

 of the animal from end to end (Fig. 8). The body of a man 



is a sack closed in every 

 /; Interior medium part ; the digestive tube 

 traverses this sack, giving 

 it, as we have said, the 

 general plan of a lady's 

 muff. But we must be 

 careful to notice that the 

 contents of the digestive 

 tube are external to the 

 body. When we eat we 

 do not introduce food into 

 our interior medium, but 

 into an external cavity 

 between which and the 



interior medium exchanges are set up (absorption, secretions, 

 etc.), analogous to those which go on between the lungs 

 and the blood. 



In the interior medium of multicellular beings, particu- 

 larly in the walls which separate the cells from each 

 other, deposits are made of substances (excrementitial or 

 reserves, according to the case), which are more resisting and 

 rigid than protoplasm. These resisting substances solder 

 themselves together and form a sort of network which is 

 also resistant and encumbers the whole organism, serving as 

 a support to the living parts. This is the skeleton. Its 

 importance increases in proportion as the being grows old 

 and, at each successive instant, it tends to fix the form 

 of the being. 



These remarks, however general, are sufficient to make 

 the following chapters understood. 



