90 THE RESPIRATORY FUNCTIONS 



normally stimulated by venous blood as well as by afferent 

 impulses from the lungs by way of the vagi, and inspiratory 

 and expiratory movements are thence co-ordinated. The 

 diaphragm is drawn back, the intercostal, scaleni, and other 

 muscles raise the ribs, and air enters the lungs, distending the 

 elastic walls of the air-cells. In ordinary circumstances, the 

 chest then contracts, largely passively, with little muscular 

 exertion and air is expired. Expiratory effort, although 

 scarcely realised in ordinary breathing is, however, evoked in 

 coughing and sneezing, as well as in producing vocal sounds. 

 Inspiration and expiration thus alternate, in healthy adult 

 horses at perfect rest, from twelve to sixteen times, in cattle 

 about fifteen to twenty times, in sheep from thirteen to 

 eighteen times, and in dogs from fifteen to twenty times per 

 minute. 



The respiratory centre is stimulated in various ways. 

 Changes in the air breathed, such as an increase of C0 2 , or 

 an excess of this gas in the blood, will stimulate the centre and 

 cause quicker and deeper breathing. Similarly, deficiency 

 of haemoglobin, or loss of blood by haemorrhage, increase 

 respiratory activity. Then there are various drugs which 

 directly stimulate the centre, notably strychnine, ammonia, 

 atropine, thebaine, substances of the digitalis group, and 

 caffeine. It is first excited and then depressed by chloro- 

 form, ether, and alcohol. Its activity is diminished, with 

 consequent slow and shallow respiration, by opium, eserine, 

 and aconite, and by the narcotics and depressants of the 

 central nervous system generally. The centre can also be 

 affected reflexly, for as we have seen afferent impulses from 

 the lungs and bronchioles by way of the vagus determine 

 normal respiratory rhythm. Thus any irritant gas inhaled, or 

 irritant drug excreted by the bronchial mucous membrane, 

 will affect the centre, deepen respiration and may cause 

 coughing or sneezing. Other sensory (afferent) impulses, 

 such as cold to the skin, a blow, or pain anywhere, usually 

 stimulate the respiratory centre, often after a momentary 

 inhibition with holding of the breath. 



When respiration is paralysed, as in narcotic poisoning, 

 subcutaneous injection of strychnine is sometimes useful. 

 The changes in calibre of the bronchial tubes are determined 



