192 PHYSIOLOGY FOR DENTAL STUDENTS. 



before the experiment. In the meanwhile the vasoconstrictor 

 fibers die, and dilators remain alive, so that their action 

 can be shown by taking a volume record of the limb before and 

 during the stimulation of the nerve. 



Vasomotor Reflexes. In the same manner that the heart is 

 influenced by afferent stimuli reaching cardiac centers from peri- 

 pheral parts of the body, we find afferent stimuli affecting the 

 size of the blood vessels reflexly by way of the vasomotor center 

 located in the medulla near the vagus center and the vaso- 

 motor nerves. Some of the afferent impulses cause dilation of 

 the blood vessels, while others cause constriction. Perhaps the 

 most important of the sensory nerves, which, when stimulated, 

 produce a dilation of the blood vessels, is the cardiac depressor, 

 which we mentioned in connection with the afferent nerves of 

 the heart. It will be remembered that this nerve has sensory 

 endings in the left ventricle and in the aorta, and that these are 

 stimulated when the blood pressure in the arterial system reaches 

 too great a height for the safety of the individual. The stimuli 

 originating in the sensory endings of this nerve are carried to 

 the cardiac center and are then transmitted to the heart through 

 the vagus nerves. Besides the slowing of the heart which is thus 

 produced, there also occurs a dilation of the peripheral vessels 

 brought about by the action of the stimuli on the vasomoter 

 center. This is easily demonstrated by electrically stimulating 

 the cardiac depressor nerve after both vagus nerves have been 

 cut in the neck and the reflex vagus action thus removed. The 

 fall of blood pressure which is obtained under these conditions 

 is due to an inhibition of the constrictor center and a stimula- 

 tion of the dilator center of the vasomotor nerves. 



The stimulation of many of the afferent or sensory nerves of 

 the body is followed by a change in the blood pressure. Just 

 what this change may be it is often impossible to predict. Strong 

 sensory stimuli of short duration may produce a marked rise in 

 blood pressure, the constrictor center being the most affected. 

 On the other hand, if the stimuli are very strong or continued 

 over a long period of time, the constrictor nerves may become 

 exhausted, as it were, resulting in a dilation of the arteries and 



