188 PHYSIOLOGY FOR DENTAL STUDENTS. 



sor. It has its beginnings in filaments lying in the left ventricle 

 and in the aorta, and runs to the medulla in the vagus trunk, in 

 most mammals, or as a separate nerve in the rabbit. Under ordi- 

 nary conditions, cutting this nerve produces no effect on the 

 heart beat, but stimulation of the upper end of the cut nerve, 

 i. e., the end running to the head, results in a marked slowing of 

 the heart and fall in the blood pressure. If the experiment is 

 repeated after cutting the vagi, the heart is slowed, but the fall 

 in blood pressure, though less evident, still occurs. The normal 

 stimulus to the depressor nerve is a high blood pressure in the 

 ventricles and aorta. The stimulus, thus set up, acts through the 

 vagus center and the vagus nerve, and slows the heart. It also 

 acts on the vasomotor center and causes the blood vessels to 

 dilate. Both changes produce a fall in the blood pressure. The 

 vagus nerve, besides the afferent vagus fibers, carries afferent or 

 sensory nerves to the vagus center. This can be demonstrated 

 by cutting one vagus and stimulating the central end, i. e., the 

 end running to the brain. A marked slowing of the heart usually 

 results. By acting through the vagus center and nerves, or 

 through the sympathetic center and nerves, most of the sensory 

 nerves of the body, if stimulated, can produce a reflex slowing 

 or quickening of the heart beat. One cannot, however, always 

 predict exactly what result will be obtained. The stimulation of 

 the fifth nerve in the nasal cavity or in the mouth always causes 

 a reflex slowing of the heart. Stimulation of the laryngeal nerve 

 and the nerves of the peritoneum have a similar effect. It 

 is also of interest to note that the act of swallowing will often 

 cause a decrease in the rate of the heart through reflex vagus 

 action. 



The relation of the blood pressure to the rate of the heart has 

 been noted in connection with the cardiac depressor nerve 

 (p. 187). Anything which produces an increase in the pulse 

 rate, other conditions being equal, will cause an increase in the 

 blood pressure, and this acts reflexly to bring about a slowing 

 of the heart, The reverse of this is likewise true. In this quick- 

 ening or slowing of the heart, the vagi and the sympathetic 

 nerves always act. In the adult the normal rate of the heart 



