298 ELEMENTS OF PHYSIOLOGY 



glands to make their secretions, and the muscles of the 

 arteries and intestines to contract. They do this as a 

 reflex response to the impulses going to the spinal ganglia 

 from the sensory nerves of the system. The pressure of 

 food in the stomach excites the flow of the secretions. At 

 a flash of bright light the eye winks and the pupil con- 

 tracts. The sweat glands secrete under the influence of 

 warmth. It was formerly believed by physiologists that 

 the sympathetic ganglia themselves sent the reflex motor 

 impulses, but it is now believed that these come from the 

 spinal ganglia. The sympathetic ganglia are supposed to 

 reenforce the current and aid in the nutrition of the nerves 

 that pass through them. 



552. The Brain has Considerable Influence over the 

 sympathetic nerves, but in an indirect way. We can- 

 not control the digestion or the heart beat by the will, yet 

 the thoughts and emotions may powerfully affect them. 

 The face blushes from dilatation of its blood vessels when 

 one is embarrassed or self-conscious. It becomes pallid 

 with fear. Sorrow depresses the activity of the cord, 

 which in turn sends out fewer orders, so that the food is 

 not digested and the cells are not nourished. Yet to 

 guard against too great interference by the emotions and 

 the mind, nature has arranged that impulses starting at 

 the brain must pass through at least two sets of ganglia 

 before they can affect nutrition and circulation. The 

 examination of the food is carried on by the higher senses 

 and the brain, for it is important that hard things and 

 poisonous substances should not be swallowed. But the 

 regulation of the flow of the saliva is left to the gray 

 matter at the top of the spinal cord ; the secretion of the 

 saliva is a reflex act excited by the taste of the food, and 

 does not enter into consciousness. 



553. Strong influences are needed to impair the action 

 of the sympathetic nerves and ganglia. Poisons which 



