322 OF THE CIRCULATION OF THE BLOOD. 



Glaisher, in his scientific balloon ascents (see his article on Aeronautics in 

 the Encyclopaedia Britannica, 9th edit., 1875, p. 197), found that the pulse, 

 which was 76 before starting, increased considerably at high altitudes, being 

 90 at 10,000 feet, 100 at 20,000 feet, and 110 at greater heights. 



249. It may be advisable briefly to sum up the circumstances affecting the 

 frequency of the heart's action. As has already been pointed out, the heart 

 not only possesses an internal system of ganglia and nerves by which its 

 movements can be maintained and regulated for some time after excision 

 from the body, but it is also under the control of centres situated in the 

 medulla oblongata ; and as there is reason for believing that there are both 

 accelerating and inhibitory centres in the heart and in the medulla, it is 

 obvious that great difficulty must exist in determining the cause of any par- 

 ticular variation in the frequency of its beats that may be observed. We 

 have, however, evidence that the heart may be influenced 



A. By conditions affecting the intraoardiae motor ganglia, which may be 

 (1) excited, and the frequency of the pulsations increased a, by heat, b, by 

 mechanical, chemical, and electrical excitation, c, by increased blood-pres- 

 sure, which is essentially a mechanical irritant, d, by various poisons ; or (2), 

 depressed, and the pulsations lowered a, by cold, b, by diminished blood- 

 pressure; c, by certain poisons, as chloral. 



B. By conditions affecting the intracardiac inhibitory ganglia, these being 

 stimulated and the heart slowed or stopped in diastole by certain poisons 

 (Muscarin) and paralyzed by others (Atropin). 



c. By conditions affecting the intracardiac accelerator ganglia, leading to 

 increased frequency of action. At present these cannot be satisfactorily 

 distinguished from those affecting the iutracardiac motor ganglia. 



D. By conditions affecting the cardiac inhibitory centres in the Medulla 

 oblongata, such as increased blood-pressure, which acts directly upon them, 

 or by irritation of the depressor nerve, or of various other sensory nerves, 

 which act upon them in a reflex manner, or by the action of poisons (Digi- 

 talis), all of which excite these centres and cause slowing of the heart's 

 action. 



E. By certain conditions affecting the accelerator centres in the medulla, 

 as diminished blood-pressure, mental processes, the action of poisons. They 

 may also probably be excited in a reflex manner. It will be seen from the 

 above that certain conditions, increased blood-pressure for example, exerts 

 a double action on the cardiac nerves. On the one hand, it excites the heart 

 to increased frequency of action by direct irritation of its motor ganglia 

 through filaments distributed to the Endocardium ; on the other hand, it 

 stimulates the vagal centres in the Medulla, and thus causes slowing of the 

 heart, the actual result in any given case being dependent upon the prepon- 

 derating influence of the cardiac motor or medullary inhibitory centres. 

 Usually the medullary centres are strongest, and increased blood-pressure 

 causes slowing of the heart ; but if the vagi be divided, so that they can no 

 longer act on the heart, increased blood-pressure stimulates the heart to more 

 rapid action. The increased rapidity of the pulse observed by Asp to fol- 

 low excitation of the central extremity of the divided spinal cord lias not 

 been found to be constant by Dittmar, 1 the variation being probably depen- 

 dent upon the complicated effects of the increased blood-pressure, which the 

 latter experimenter has shown results from this operation. 



1 Ludwig's Arbeiten, 1871, p. 18. 



