514 OF RESPIRATION. 



matism, pleuritis, pericarditis, or other painful disorders of the parts forming 

 the parietes of the thorax. 



548. We have now to inquire into the mode in which the Muscular move- 

 ments of Respiration are kept up by nervous power. There can be no doubt 

 that these movements, though partly under the control of the Will, are essentially 

 " automatic" in their nature. Their chief centre is the upper part of the 

 Medulla Oblongata^ into which may be traced the principal excitor nerves that 

 convey the stimulus on which the movements are dependent, whilst from it 

 proceed the principal motor nerves by which they are carried into effect. And 

 thus it happens that the whole of the Encephalon may be removed from above, 

 and the Spinal cord (as far up as the origin of the phrenic nerve) from below, 

 without suspending the most essential of the respiratory movements. But other 

 parts of the automatic centres are concerned in the ordinary movements of re- 

 spiration ; and there is probably no part that may not be excited to action, by 

 the extraordinary stimulus which results from a prolonged interruption to the 

 aeration of the blood ( 546). The chief " excitor" of the respiratory move- 

 ments is unquestionably the Pneumogastric nerve. When this is divided on 

 both sides, according to. the experiments of Dr. J. Reid, 1 the number of respi- 

 ratory movements is considerably diminished, usually about one-half. Now if 

 this nerve excites the motions of respiration by its powerful action in producing 

 sensation, we should expect to find its trunk endowed with considerable sensi- 

 bility, which is not the case ; for all experimenters agree in stating that, when 

 its trunk is pinched or pricked, the animal does not exhibit signs of pain nearly 

 so acute, as when the trunks of the ordinary spinal nerves, or of the fifth pair, 

 are subjected to similar treatment. It cannot be questioned, however, that its 

 power as an excitor of respiration is very great ; since, besides the fact of the 

 diminution in the number of inspirations which occurs immediately on section 

 of it, irritation of its trunk in the neck is instantly followed by an act of in- 

 spiration. It is evident that this power must arise from impressions made upon 

 its peripheral extremities. The impression is probably due to the presence of 

 venous blood in the capillaries of the lungs ; or, as Dr. M. Hall thinks, to the 

 presence of carbonic acid in the air-cells. Either or both may be true. The 

 Pneumogastric nerve, however, is not the only excitor of the respiratory move- 

 ments 'j since, when the nerve is cut on each side, they still continue, though 

 with less frequency. The removal of the Encephalon diminishes the frequency 

 of the respiratory movements, whether it be performed before or after the section 

 of the Vagi. Dr. Reid found that in a kitten of a day old, in which the 

 inspirations had been 100 per minute, they fell to 40 when the Encephalon was 

 removed; and on subsequently cutting the Pneumogastrics, the number of 

 inspirations instantly fell to between 3 and 4 in the minute, and continued so 

 for some time. Hence it has been supposed that the respiratory movements are 

 partly dependent upon sensation, a motor influence being excited by it ; but 

 it may be fairly surmised, from the close dependence of nervous activity upon 

 the oxygenation of the blood, that a " besoin de respirer" may originate in the 

 circulation of imperfectly aerated blood in the nervous centres themselves, and 

 may become the direct excitor of respiratory movements. 



549. But why (it may be asked) do the movements continue, when the 

 Pneumogastrics have been divided, and the Encephalon has been removed ? It 

 is evident that there must be other exciters to the action of the respiratory 

 muscles. Amongst these, the nerves distributed to the general surface, and 



1 "Edinb. Med. and Surg. Journ.," vol. li. ; and "Phys., Anat., and Pathol. Res.," 

 p. 177. Dr. Reid has satisfactorily shown the statement of many experimenters, that the 

 inspirations are increased in frequency after this operation, to be erroneous ; this idea hav- 

 ing originated in their very prolonged and laborious character. 



