204 THE ENGINES OF THE HUMAN BODY 



alter and affect its movements. The stomach, too, has 

 its own movements, but, as we have already seen, it also 

 can be affected by the emotional conditions which may 

 affect our brains. Grief can make the bowels yearn. 



The arterioles which open into the capillary fields in 

 the wall of the bowel are under the direct control of the 

 special nerve centres which regulate the distribution of 

 blood to the various parts of the body. During digestion 

 the bowels are flushed with blood ; the stopcock mechan- 

 ism of the arterioles is turned full on. In hard muscular 

 exercise the opposite is the case ; they are turned off to 

 an extent sufficient for providing an ample supply to 

 the muscles. 



Lastly, we have to consider the manner in which the 

 products of digestion are absorbed from the chyme. 

 Here we see Nature adopt a method or contrivance of 

 a kind well known to engineers. In order to raise a 

 head of steam in an engine as quickly and as economically 

 as is possible, the heat of the furnace is carried through 

 flues or pipes which are immersed in and surrounded 

 by the water of the boiler. The engineer aims at bringing 

 an extensive and thin sheet of water in contact with an 

 equally extensive heating surface. In our lungs Nature 

 has brought a film of blood 27 feet square against an 

 equally extensive ventilating surface. In the small bowel 

 an absorbing surface of 8 square feet is brought in 

 contact with a constantly changing film of chyme. The 

 absorbing surface is really greater than I have estimated, 

 because of two circumstances. In the upper part of the 

 bowel — the jejunum — the lining membrane is raised into 

 crescentic folds which cross the lumen of the bowel and 

 almost double its absorbing surface (fig. 41). Then there 

 is from the beginning to the end of the small bowel a 

 carpet of villi — miniature projecting fingers or tongues — 

 over which the chyme has to flow. Villi contain extensions 

 of the capillary blood-field and also a slip of muscle which 

 gives them a power to contract and thus act as pumps. 

 Within them, too, are the commencement of lacteal 

 vessels. The absorbing surface may be about twice the 



