AIR AND RESPIRATION. 479 



respiratory movements. It must also be carefully noted in this 

 connection that diminished oxygenation of the blood, whilst 

 increasing the respiratory activity, stimulates the other two 

 great centres in the medulla, increasing the arterial resistance 

 through the vaso-motor centre, and slowing the heart through 

 the cardiac centre. 



The afferent impressions from the lungs and respiratory 

 passages, besides falling into the respiratory centre, also reach, 

 if sufficiently powerful, the convolutions, where they are felt as 

 various sensations, referred more or less accurately to the re- 

 spiratory organs. In health these sensations of common 

 sensibility are feeble ; and we do not appreciate them until 

 they are converted into sensations of pain, oppression, distress, 

 or irritation, in disorder or disease. 



Amongst the nerves of the respiratory muscles one group 

 demands special notice, viz. those distributed to the bronchi. 

 These are motor filaments of the vagus, which originate in the 

 respiratory centre and supply the muscles regulating the 

 calibre of the air-tubes. They bring the bronchi under the 

 control of the medulla, and thus of the afferent impressions, 

 especially of those very impressions which originate in the respi- 

 ratory passages, the seat of their own distribution. 



II. PHARMACODYNAMICS. 



The extensive relations of the respiratory organs to the 

 external air, to the blood and circulation, and to the nervous 

 system, afford us abundant means of influencing their mode of 

 action. These means we will now review in their natural 

 physiological order : 



1. The Air. The air which comes in contact with the 

 organs of respiration may be altered in five different respects, 

 each of which will have a physiological effect upon the functions 

 of the lungs, viz. as regards (a) its absolute amount, (b) its chemi- 

 cal composition, (c] its temperature, (d] its moisture, and (e) its pres- 

 sure. 



(a) The supply of air, like that of the food, may be entirely 

 arrested for a time, another gas with different physiological 

 properties, such as Nitrous Oxide, being allowed to take its 

 place. Or the amount respired may be simply reduced, by ad- 

 ministering rarefied air ; or increased, by admitting oxygen or 

 compressed air into the lungs. The same effects may be pro- 

 duced by ordering little or much muscular exercise respec- 

 tively. 



(b) The chemical composition of the atmosphere, physiologi- 

 cally speaking, relates only to the amount and quality of the 



