THE VASOMOTOR NERVES. 189 



varies between 68 and 76 per minute. In children the rate is a 

 little faster, and in infants it may be normally 130 or more. 



The Nervous Control of the Blood Vessels. 



During muscular activity the metabolism of the body may be 

 increased five or six times, as can be judged from the amount of 

 carbon dioxide given off by the lungs. Since this increase is due 

 to the activity of the muscles, it is necessary that these obtain 

 a greater supply of oxygen, and that they be able to rid them- 

 selves of the carbon dioxide which is a waste product of their 

 activity. Every other organ requires an increased blood supply 

 when it becomes active, so that blood has to be diverted from the 

 inactive to the active tissues, and the least important activities 

 of the body have to be subordinated to the one which is mast 

 needed at the time in question. This action is brought about 

 partly by the central nervous system, acting through its afferent 

 and efferent nerves on the musculature of the blood vessels of 

 the body, and partly by means of chemical substances which are 

 produced at an early stage of the activity itself. 



The Vasomotor Nerves. It was discovered in the middle of 

 the past century by the French physiologist, Claude Bernard, 

 that section of the cervical sympathetic nerve in the neck of the 

 rabbit causes a marked dilation of the blood vessels of the ear, 

 and that during stimulation of the nerve with an electric cur- 

 rent, the blood vessels become very small, and the ear conse- 

 quently colder. This experiment shows that the nervous system 

 plays an important role in the control of the flow of blood through 

 the tissues, and from it many important truths about the nervous 

 control of the blood vessels may be deduced. If cutting a nerve 

 will cause the blood vessels to dilate, and stimulating the same 

 nerve with an electric current will cause the vessels to constrict 

 to much less than their normal size, it follows that the blood 

 vessels must be normally held in a state half way between ex- 

 treme dilation and constriction by stimuli received from the 

 nervous system. The nerve fibers which carry the stimuli, be- 

 cause of their power of producing constriction of the blood ves- 

 sels, are known as vasoconstrictor nerve fibers. They are com- 



