IS THE BODY A MACHINE? 37 



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 

 which passed into the blood in the intestine 

 through the influence of the osmotic force, now 

 pass out into the lymph under the influence of the 

 same force. The food is thus brought into the 

 lymph ; and since the lymph lies in actual contact 

 with the living muscle fibres, these fibres are now 

 able to take directly from the lymph the material 

 needed for their use. The power which enables 

 the muscle fibre to take the material it needs, 

 discarding the rest, is, again, one of the vital pro- 

 cesses which we defer for a moment. 



Respiration. Pursuing the same line of study, 

 we turn for a moment to the relation of the circu- 

 latory system to the function of supplying the body 

 with oxygen gas. Oxygen is absolutely needed 

 to carry on the functions of life; for these, like 

 those of the engine, are based upon the oxidation 

 of the fuel. The oxygen is derived from the air 

 in the simplest manner. During its circulation 

 the blood is brought for a fraction of a second 

 into practical contact with air. This occurs in the 

 lungs, where there are great numbers of air cells, in 

 the walls of which the blood-vessels are distrib- 

 uted in great profusion. While the blood is in 

 these vessels it is not indeed in actual contact 

 with the air, but is separated from it by only a 

 very thin membrane so thin that it forms no hin- 



