160 



ADVANCED PHYSIOLOGY 



-a 



when the muscles of the arms or legs are working they need 

 more blood than usual. Hence the vaso-motor nerves cause 

 the blood vessels of the muscles to relax and thus allow more 

 blood to be supplied to them. The consequence is that less 



blood is at liberty to 

 go to the cells in the 

 stomach and intes- 

 tinal walls, when 

 these should be busi- 

 ly secreting diges- 

 tive juices or passing 

 along the food by 

 peristaltic contrac- 

 tions. Thus, with 

 too little blood to 

 effect digestion the 

 food is sluggishly 

 handled, secretions 

 are less in amount 

 than they need be, 

 and the food is not 

 sufficiently change( 

 to allow of read} 

 absorption. 



Blushing and 

 turning pale aro evi- 

 dences of the work 



of this system. These changes are due entirely to the differ- 

 ence in the size of the blood vessels of the face and are 

 brought about through the influence of nerves; Fig. 90. 



Control of the Vaso-Motor System. These messages to the 

 blood vessels go out from the brain without any consciousness 

 on our part, however; in fact we cannot control them, as is 

 shown by the vain struggles of people to keep from blushing 

 vhen they are embarrassed. This is a most fortunate pro- 



FIG. 90. APPARATUS FOR DEMONSTRATING 

 THE FUNCTION OF THE VASO-MOTOR SYSTEM 

 The arm is placed in the glass cylinder which is filled 

 tfith water. The tube a connects with the cyl- 

 inder also. Evidently any swelling of the arm will 

 cause the level of the water in the tube to rise. 

 Having noted, by the marker, the height of the 

 water, the person whose arm is in the cylinder is set 

 to studying hard. The marker quickly falls, show- 

 ing that blood has left his arm to go to the aid of 

 the hard working brain. If he stops studying the 

 marker rises again. (Fick) 



