THE CIRCULATION 237 



that the relaxation of vaso-constriction occurs in a reflex 

 manner. The reflex relaxation of the splanchnic area is a 

 matter of great importance, because it can be brought about 

 by stimulation of one of the sensory nerves of the heart. The 

 higher the blood-pressure, the harder the heart would work if 

 left to itself. It is an impetuous organ, always trying to 

 quicken its pace and to increase the force of its beat. Ex- 

 cessive zeal would get it into trouble if severe precautions were 

 not taken to hold it in check. True, it is encouraged by certain 

 " accelerator nerves " sympathetic filaments which leave the 

 spinal cord by the anterior roots of the second and third 

 thoracic nerves ; but the influence which the accelerators exert 

 under normal conditions is not, it would seem, very pronounced. 

 The nerves which restrain the heart are much more in evidence 

 than those which urge it on. The arrangements for diminishing 

 the work of the heart are of two kinds. In the first place, 

 branches derived from the vagus act as a continuous check. 

 From a certain spot in the medulla oblongata, the cardio-inhibi- 

 tory centre, impulses are always descending to slow the heart. 

 They are of reflex origin, but a high blood-pressure in the centre 

 increases the facility with which they are transmitted. Some 

 of these stimuli originate in the heart itself, ascending and 

 descending the vagus nerve. The remainder come from various 

 sources. A severe injury to any part of the body slows the 

 heart. Injury to the intestines, such as occurs in peritonitis, 

 is particularly effective in increasing vagus inhibition. Slowing 

 of the heart lowers blood- pressure. When both vagi are cut, 

 the heart begins to gallop whatever may be the pressure against 

 which it has to work. 



A sensory nerve of the heart, termed the " depressor," is the 

 chief agent in lowering blood- pressure. Its course is not the 

 same in all animals, but it runs more or less in conjunction with 

 the vagus. Usually it joins its superior laryngeal branch. Im- 

 pulses which ascend this nerve inhibit the constriction of the 

 splanchnic vessels. They open a floodgate which brings down 

 the general pressure. The severe pain and extreme distress of 

 angina pectoris are the cry of the heart when blood-pressure is 

 too high when it feels unable to work against it. This was 

 recognized by physiologists long before a remedy was known. 

 A systematic search was instituted for a drug which could be 



