364 RESPIRATION [CH. XXIV. 



It is probable that of the two sets of impulses, those which are 

 started by the inspiratory movement play a more active part in the 

 regulation of respiration than those started by the expiratory move- 

 ment. Gad explains the latter by supposing they are simply due to 

 a cessation of the former, or, in other words, that there only exists 

 one class of afferent fibres in the vagus concerned 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 respira- 

 tion ceases for a short time. This follows naturally from the experi- 

 ments previously described. This can be done on an animal with a 

 pair of bellows fixed to a tube in the trachea ; or voluntarily by one- 

 self taking a number of deep breaths rapidly. This condition, called 

 apncea, 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, however, 

 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, 

 apnosa cannot obviously be due to such reflex action. In such de- 

 pressed 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 in- 

 spiration, followed by an expiration ; but the latter, instead of being 

 easy and uninterrupted, as in normal breathing, is obstructed, the 

 glottis being momentarily closed by the approximation 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 out- 

 streaming 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, 



