320 ANATOMY AND PHYSIOLOGY 



ceral muscles, secreting cells, vessel walls, are all under the immediate 

 domain of the sympathetic ganglia and nerves, whose motor and 

 sensory fibers are parts of the great nerve system of the body, 

 through communicating branches. 



Certain facts indicate a communication between the brain and 

 sympathetic nerves, for instance, the thought of food causes a flow 

 of saliva; think of a lemon salivary cells are stimulated. Fright 

 or anxiety may inhibit or prevent the secretion of saliva; or inter- 

 fere with digestion through a similar effect upon other digestive 

 fluids; and it is well known that the secretion of milk is greatly 

 modified by mental or emotional influences. 



So with general mso-motor action. We all know the blanched 

 face of fright or mental shock; the flush of joyous excitement; or 

 the blush of embarrassment. All of these are sympathetic reflexes 

 of psychic origin. In the case of secretion of digestive fluids, the 

 psychic flow follows the thought of food at once; after that comes 

 the secretion caused by the presence and contact of food in the 

 different parts of the alimentary tract. 



Again the effect of vaso-motor action may be seen when intense 

 cold is applied to the skin. The cutaneous vessels contract, the 

 blood is driven out, the skin becomes white. The opposite condi- 

 tion is caused by heat the vessels dilate, the blood flows in and 

 the skin is red. 



By alternate action of the two kinds of vaso-motor nerves 

 (vaso-dilators and vaso-constrictors) , the blood supply is adapted to 

 special and varying needs of different parts of the body, and the 

 balance of pressure preserved in their vessels. 



When an organ has work to perform its vessels dilate and the 

 necessary blood is supplied. When the work is finished the vessels 

 return to their usual size (their vessel tone being restored by vaso- 

 constrictors). 



The process of digestion, for example, requires that there should 

 be much blood in many organs; the same is true of general muscular 

 exercise. Consequently, to exercise violently after a full meal is a 

 mistake, because the muscles would deprive the digestive organs 

 of the extra blood which they need, and an attack of indigestion 

 might follow; at best, digestion would be delayed. It would be 

 better to delay the exercise. 



Many examples might be given and will probably occur to the 



