THE INTEGRATIVE ACTION OF THE VASCULAR SYSTEM 225 



sets of nerves, the vagus nerves which hold the heart rate 

 in constant check and the sympathetic nerves which make 

 the rate faster. Interestingly, the constant moderate action 

 of the vagus nerves can be used to speed up the heart; it is 

 only necessary to remove the check which they exert. The 

 blood vessels, especially the arterioles, can be made smaller 

 here and larger there, also by action of the sympathetic 

 nerves, thus hmiting the flow to one part and distributing 

 a larger volume to another part as need arises; indeed, the 

 mass of the blood can be largely shifted from one region 

 of the body to other regions in special emergencies. 



The Constancy of the Internal Environment. One of the 

 most striking features of the more highly developed organ- 

 isms is their independence of their surroundings. They can 

 go long without water and food, they can endure extremes 

 of outer temperature, they can Hve equally well at the 

 seashore or on the mountain tops. Lower organisms have 

 not these capacities. They have been developed by remark- 

 able automatic arrangements whereby, in spite of external 

 changes and in spite of bodily activities which tend to 

 disturb the internal conditions, the fluid matrix of the body 

 is kept constant. The great French physiologist, Claude 

 Bernard, stated fifty years ago that it is the fixity of the 

 milieu interieur which is the condition of free and independent 

 hfe. "All the vital mechanisms," he wrote, "however varied 

 they may be, have only one object, that of preserving 

 constant the conditions of Hfe in the internal environment." 

 We shall now examine some of the ways in which this con- 

 stancy is maintained. 



The Constancy of Blood Sugar. Grape sugar or glucose 

 is the form into which starchy food is changed for use in 

 the body. Of all energy-yielding materials supphed by the 

 food, glucose is the most readily serviceable. When it is 

 provided in abundance it is preferably utilized; the burning 

 of fat is almost completely stopped. Furthermore, according 

 to present views, glucose or its storage precursor, glycogen, 

 is essential for muscular contraction. The substance is 

 continually being used, therefore (even during sleep the 

 heart muscle and the muscles of respiration are consuming 

 glycogen) and it can be renewed only periodically. How is 



