THE STORY OF THE LIVING MACHINE. 



blood in the arteries is under pressure, while 

 there is little or no pressure in the veins. Into 

 the 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 distribution of the 

 blood explained upon me- 

 chanical principles, but the 

 supplying of the active parts 

 of the body with food is in 

 the same way intelligible. 

 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 in- 

 stance, the muscles it is 

 again p.laced under conditions 

 where osmosis is sure to oc- 

 cur. In the muscles the thin-walled blood-vessels 

 are surrounded and bathed by a liquid called lymph. 



FIG. 6. A bit of muscle 

 with its blood-vessels : 

 a, the muscle fibres ; b, 

 the minute blood-ves- 

 sels. The fibres and 

 vessels are bathed in 

 lymph (not shown in 

 the figure), and food 

 material passes through 

 the walls of the blood- 

 vessels into this lymph. 



