48 THE STORY OF LIFE'S MECHANISM. 



details of this matter we need not go, but this 

 will be sufficient to indicate that the whole pro- 

 cess is a mechanical one. 



We must not fail to see, however, that in this 

 problem of circulation there are two points at 

 least where once more we meet with that class of 

 phenomena which we still call vital. The beating 

 of the heart is the first of these, for this is active 

 muscular power. The second is a contraction of 

 the smaller blood-vessels which regulates the blood 

 supply. Both of these phenomena are phases of 

 muscular activity, and will be included under the 

 discussion of other similar phenomena later. 



We next notice that not only is the distribu- 

 tion of the blood explained upon mechanical 

 principles, but the supplying of the active parts 

 of the body with food is in the same way intelli- 

 gible. As we have seen, the blood coming from 

 the intestine contains the food material received 

 from the digested food. Now when this blood 

 in its circulation flows through the active tissues 

 for instance, the muscles it is again placed 

 under conditions where osmosis is sure to occur. 

 In the muscles the thin-walled blood-vessels are 

 surrounded and bathed by a liquid called lymph. 

 Figure 6 shows a bit of muscle tissue, with its 

 blood-vessels, which are surrounded by lymph. 

 The lymph, which is not shown, fills all the 

 space outside the blood-vessels, thus bathing both 

 muscles and blood-vessels. Here again we have 

 a membrane (i.e. the wall of the blood-vessel) 

 separating two liquids, and since the lymph is of 

 a different composition from the blood, dialysis 

 between them is sure to occur, and the materials 



