THE RESPIRATORY SYSTEM. 2/7 



due to the stimulation of the endings in the lungs of the afferent fibres 

 of the vagus which inhibit inspiration. 



Apnoea also occurs during deglutition, and lasts for a period of 

 about 6 seconds ; the afferent impulses pass along the glossopharyngeal 

 nerve. The effect of this inhibition is to prevent particles of food 

 being drawn into the lungs by an inspiratory act during the process 

 of swallowing. 



Another interesting example of the adaptation of the respiratory 

 mechanism to the needs of the animal is seen in ducks. When a duck 

 plunges its head into water in search of food, the respiratory move- 

 ments are inhibited ; and this form of apnoea can be reproduced 

 experimentally by placing the duck in the vertical position with its 

 head downwards. The afferent impulses which travel to the respiratory 

 centre and inhibit respiration arise in the muscles of the neck and in 

 the labyrinth of the ear after section of the afferent nerves from these 

 muscles or destruction of the labyrinth, this form of apnoea can no 

 longer be evoked. 



CHEYNE-STOKES BREATHING. 



This form of breathing (fig. 1 1 2), which is not infrequently observed 

 in human beings living at high altitudes or suffering from various 

 diseases, more especially those affecting the circulatory system, has the 

 following characters. After an apnoeic pause respiration begins, the 

 breaths being shallow at first and gradually increasing in depth till 

 they reach a maximum. They then become smaller, and in a short 

 time cease altogether, being succeeded by a period of apnoea. Cheyne- 

 Stokes breathing can also be produced experimentally in healthy 

 persons as a result of prolonged forced breathing, as is seen in fig. Ill; 

 the immediate effect of the forced breathing is a period of apncea, and 

 when breathing recommences, it often exhibits the periodic character 

 just described. 



The phenomenon is caused by lack of oxygen in the blood. During 

 the period of forced breathing, the tension of carbonic acid in the 

 alveolar air falls considerably, leading to prolonged apncea, during 

 which the oxygen tension in the blood sinks and the supply to the 

 respiratory centre is inadequate ; as a result lactic acid is formed in 

 the respiratory centre, and, together with carbonic acid, stimulates the 

 respiratory centre, although the tension of carbonic acid alone is 

 insufficient. Respiration begins.again, and the oxygen taken into the 

 lungs and into the blood oxidises the lactic acid. At the same time, 

 however, the deeper breathing removes some carbonic acid from the 

 lungs, the chemical stimulus to respiration again disappears, and the 



