442 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



gastric nerves (the channels for the conveyance of sensory impulses from 

 the lungs to the respiratory centre) it is very difficult to cause apnoea by in- 

 flation of the lungs with air, while if pure hydrogen is used violent dyspnoea 

 results. Tt seems, then, that apnoea cannot be produced after division of the 

 vagi unless there he an accumulation of () in the lungs. These facts suggest 

 that the frequent forced inflations of the lung- excite the pulmonic peripheries 

 of the pneumogastric nerve-, thus generating impulses which inhibit the inspi- 

 ratory discharges from the respiratory centre. This view receives further sup- 

 port in several facts: first, that the same Dumber of inflations, whether of pure 

 (). of air, or of H, causes apnoea, the only difference being the length of the 

 apnoeic pause after the cessation of artificial respiration, which pause lasts for 

 the longest period when O is used, and for the shortest period, or not at all, 

 when II is employed; second, that apncea cannot be caused by inflation of the 

 lungs with H if the pneumogastric nerves be previously divided ; third, that the 

 arrest of respiration which occurs during swallowing (" deglutition-apnoea") 

 i- due to an inhibition of the respiratory centre by impulses generated in the 

 terminations of the glosso-pharyngeal nerves (p. 462). It therefore seems evi- 

 dent that apncea may be due to either gaseous or mechanical factors, or to both, 

 the former being effective, not because of the blood being saturated with O, 

 but because of the increased amount of O in the alveoli — a quantity sufficient 

 for a time to aerate the blood ; while the mechanical factors give rise to inhibi- 

 tory impulses which suspend for a longer or shorter period the rhythmical 

 inspiratory discharges from the respiratory centre, doubtless by depressing the 

 irritability of this centre (p. 455). From the experiment quoted it seems that 

 the first of these factors may alone be sufficient to cause apncea, but that apncea 

 is more easily produced, and lasts longer, when both factors act together, as is 

 usually the case. 



The form of hyperpncea due to museular activity is owing to the action 

 upon the respiratory centre of certain substances which are formed in the 

 muscles during contraction and are given to the blood. Muscular activity, 

 as is well known, is accompanied by an increase in the rate and depth of the 

 respiratory movements, and when the exercise is violent more or less marked 

 dyspnoea may occur. Some physiologists have been led to the belief that the 

 respiratory centre is connected directly or indirectly with the muscles by 

 means of afferent nerve-fibres which convey impulses to the centre and thus 

 excite it to activity; while other- have regarded a diminution of O and an 

 increase ofCO a in the blood a- the cause, the active muscles rapidly consum- 

 ing tin () in the blood and giving off C0 2 in great abundance. But Mathieu 

 and I'rbain. and (leppert and Zunt/..' have found that the volumes percent, 

 in the blood of O may be increased, and the volume per cent, of C0 2 decreased, 

 during muscular activity. It i- probable that the hyperpncea is due to prod- 

 ucts of muscular activity which are given to the blood and which act as 

 powerful excitant- to the respiratory centre. The precise nature of the 

 bodies is unknown, but it i- probable that they are of an acid character, for 

 1 Arehivftlr du gesammle I . 1 — , Bd. 42, S. 189. 



