OH. XXIV.] APNffiA. 36l 



that there only exists one class of afferent fibres in the vagus con- 

 cerned in respiration. This view has not, however, met with general 

 acceptance, and is against the mass of experimental evidence. 



Apnoea. If positive and negative ventilation are used 

 together rapidly and alternately at a rate quicker than the 

 respiratory rhythm, both inspiratory and expiratory processes are 

 inhibited, and the respiration ceases for a short time. This 

 follows naturally from the experiments previously described. This 

 can be done on an animal with a pair of bellows fixed to a tube 

 in the trachea ; or voluntarily by oneself, taking a number of 

 deep breaths rapidly. This condition, called apnoea, is not due, 

 as at one time supposed, to over-oxygenation of the blood, but 

 is produced reflexly. Under normal circumstances arterial blood 

 is always fully oxygenated. It is observed if inert gases, like 

 nitrogen or hydrogen, are used instead of air. The pause, how- 

 ever, is then shorter, as the blood becomes venous, and in a short 

 time stimulates the respiratory centre to activity. 



Under abnormal circumstances, namely, after division of the 

 vagi, apncea cannot obviously be due to such reflex action. In 

 such depressed conditions of the respiratory centre, the blood 

 becomes more venous than normal, and then the rapid inflation 

 of the lungs with air will produce an apnoeic condition. Fredericq 

 still holds that ordinary apnoea has a chemical rather than a 

 nervous origin. He attributes it, however, not to over-oxygenation, 

 but to a lessening of the carbonic acid in the blood. 



Special Respiratory Acts. 



Coughing. In the act of coughing there is first of all a deep 

 inspiration, followed by an expiration ; but the latter, instead of 

 being easy and uninterrupted, as in normal breathing, is ob- 

 structed, the glottis being momentarily closed by the approxima- 

 tion of the vocal cords. The abdominal muscles, then strongly 

 acting, push up the viscera against the diaphragm, and thus 

 make pressure on the air in the lungs until its tension is sufficient 

 to noisily open the vocal cords which oppose its outward passage. 

 In this way considerable force is exercised, and mucus or any 

 other matter that may need expulsion from the air-passages is 

 quickly and sharply expelled by the outstreaming current of air. 

 The act is a reflex one, the sensory surface which is excited being 

 the mucous membrane of the larynx, and the superior laryngeal 

 nerve is the afferent nerve ; stimulation of other parts of the 

 respiratory mucous membrane will also produce cough, and the 

 point of bifurcation of the trachea is specially sensitive. Other 



