502 



ORGAN SYSTEMS OF MAN 



chronized with an impulse that follows this 

 pathway. 



Even though the heart will beat autono- 

 mously in the intact animal, its beat is care- 

 fully regulated by two sets of motor nerves 

 which carry impulses directly to the sinus 

 nod (Fig. 19-10). Branches from the va- 

 gus (cranial-sacral) may inhibit the beat, 

 whereas many branches from the thoraco- 



beat by stimulating the vagus when the 

 pressure is unnecessarily high in the aorta. 

 Such action occurs following exercise when 

 the muscles no longer need a large volume 

 of blood. A second group arises in the right 

 auricle and terminates in the cardiac center 

 ( accelerator center ) . When the large skele- 

 tal muscles are in vigorous action, blood 

 flows into the right auricle in greater 



r»m.oP O 

 mercury 



Fig. 19-11. The blood pressure falls rapidly in the arteries but declines at a much more gradual 

 rate in the capillaries and veins. It is highest when leaving the heart and lowest in the 

 large veins that empty into the heart, as indicated by this sketch. 



lumbar autonomic system in the cervical 

 region may accelerate it. These are influ- 

 enced by emotions, a fact familiar to all. 



Additional sources of regulation come 

 from two groups of sensory endings close to 

 or in the heart itself. One group originates 

 in the aortic arch and terminates in the 

 cardiac center (depressor center) in the 

 medulla, and these nerves are called de- 

 pressors because they slow down the heart 



amounts than normally. As a result the 

 walls are stretched, stimulating the accel- 

 erator nerves to make the heart beat faster 

 witli the result that the muscles receive the 

 additional blood they need. Thus, by the 

 combined action of these sets of governors, 

 the heart is able to maintain a rate that sup- 

 plies all parts of the body with an adequate 

 amount of blood under highly variable 

 circumstances. 



