158 THE HUMAN MECHANISM 



now becomes very red on account of the large amount of 

 blood flowing through them. 



There is probably some compensation for this in other 

 organs, but it is an imperfect compensation. The drowsiness 

 which is apt to come on after a hearty meal is probably an 

 indication that these compensations are not complete and 

 that owing to the fall of arterial pressure the brain is not 

 receiving its normal blood supply. 



20. Some practical applications. We may pause here to 

 consider some important practical applications of these facts. 

 While the most active secretion is in progress nothing should 

 be done which will take blood away in large quantities from 

 the stomach. Muscular exercise, for example, then as always, 

 dilates the arterioles of the muscles and skin and constricts 

 those of the digestive organs ; this is obviously an unfavorable 

 vascular condition for the act of secretion. If the meal be a 

 light one, so that comparatively little of the digestive juices 

 are required, no harm may be done by taking exercise after 

 a meal ; but where the meal is heavier it is almost always 

 unwise, especially in warm weather. Similar considerations, 

 which are likewise in full accord with experience, indicate 

 that it is unwise to eat as large meals in very warm weather 

 as in cooler weather. The larger the meal, the greater the 

 amount of gastric juice required to start its digestion; but 

 in warm weather the arteries of the stomach and intestine 

 tend to be constricted (see p. 152), so that it is difficult to 

 secure an adequate blood flow through these organs, and 

 their efficiency is to this extent impaired. 



It is sometimes stated that mental work immediately after 

 meals causes indigestion by taking blood away from the 

 digestive organs and sending it to the brain. It is very 

 doubtful, however, whether the increased blood flow to the 

 brain is secured largely at the expense of that to the diges- 

 tive organs. While instances might be cited of indigestion 

 among people who do mental work upon a "full stomach," 



