65 8 NERVOUS SYSTEM. 



reduces the number of pulsations of the heart by a reflex action ; the impression being 

 conveyed to the nerve-centres by the depressor-nerves, and the force operating directly 

 upon the heart being transmitted through efferent filaments in the trunk of the pneumo- 



gastric. 



Second, the reduction in the pressure of blood in the larger arteries is independent 

 of the efferent filaments of the pneumogastric and bears no relation to the reduction in 

 the number of cardiac pulsations. 



It now remains to explain, if possible, the mechanism of the reduction in the arte- 

 rial pressure. This question is treated by Cyon by the method of exclusion. The dimi- 

 nution in the pressure followed galvanization of the central extremities of the depressor- 

 nerves even when the heart was removed from its influence by section of both pneumo- 

 gastrics in the neck, and when all the voluntary movements and the movements of 

 respiration were abolished by poisoning with woorara. In the latter case, the circula- 

 tion was kept up by artificial respiration. Without following out the various observa- 

 tions which go to show that the influence of the depressor-nerve upon the arterial pressure 

 is independent of the force or frequency of the heart's action and is due to some cause 

 which operates upon the vessels themselves, we shall simply give the results of the 

 experiments upon the splanchnic nerves. If the abdomen be opened, and one or more 

 of these nerves be divided, the arterial pressure is immediately diminished. After this, 

 if the peripheral extremities of the divided nerves be galvanized, the pressure rapidly 

 returns to the normal standard. These experiments " demonstrate that the splanchnic 

 nerves constitute the most important vaso-motor nerves in the entire organism." This 

 point being settled, the depressor-nerves were galvanized after section of the splanch- 

 nic nerves, in some cases exaggerating the general arterial pressure by compressing 

 the aorta, and in others, leaving the aorta free. " The irritation of the depressor-nerve 

 after section of the splanchnic nerve produced still a diminution in the blood-pressure, 

 but the absolute value of this diminution is much less than it was during the irritation of 

 the depressor-nerve before the section of the splanchnic." These experiments show 

 pretty conclusively that the diminished pressure in the arterial system following stimula- 

 tion of the central ends of the depressor-nerves after division is due to a reflex action on 

 "the blood-vessels of the abdominal organs, taking place through the splanchnic nerves. 

 We are sufficiently familiar with reflex paralyzing action upon the blood- vessels through 

 the sympathetic system ; and, when we call to mind the immense extent of the abdominal 

 vascular system, we can readily understand how, if the resistance to the flow of blood be 

 diminished by paralysis of the muscular coats of the small arteries, the pressure in the 

 larger arteries would be reduced. 



Mechanism of the Influence of the Pneumogastrics upon the Action of the Heart. It 

 is useless to speculate upon the exact mechanism of the action of the pneumogastrics upon 

 the heart. Although various explanations have been presented of the effects following 

 division of the nerves in the neck, and of the opposite phenomena which attend the gal- 

 vanization of their peripheral ends, they are all more or less unsatisfactory. All that can 

 be said, in the present state of our knowledge, is, that the pneumogastrics, by virtue of 

 the communicating branches from the spinal accessories, have a direct inhibitory influence 

 upon the heart. When they are divided and the heart is removed from their influence, the 

 pulsations become more rapid. When the peripheral ends of the divided nerves are 

 galvanized, the .heart beats more slowly, or its action may be arrested by a current of 

 sufficient power. This action may also be reflex, due to an impression conveyed to the 

 centres by the depressor-nerves. 



Properties and Functions of the Pulmonary Branches, and Influence of the Pneumo- 

 trics^ upon Respiration. The trachea, bronchi, and the pulmonary structure are sup- 

 plied with motor and sensory filaments by branches of the pneumogastrics. The recurrent 

 laryngeals supply the upper, and the pulmonary branches, the lower part of the trachea, 



