CH. XXVI.] THE CHEMICAL FACTOR IN RESPIRATION 381 



physiologists consider that the action of carbonic acid is not specific, 

 but that other acids can act in the same way. As the result of 

 prolonged exercise, the carbonic acid pressure in the alveoli falls 

 greatly; nevertheless there is increased pulmonary ventilation. This 

 is because lactic and perhaps other acids are thrown into the blood, 

 which raise its total acidity (hydrogen ion concentration), see p. 395. 



Changes in the oxygen pressure within wide limits have no such 

 influence ; the normal chemical stimulus to respiration is, therefore, 

 presence of an increase of carbon dioxide, and not diminution of 

 oxygen. If these limits are exceeded, as when the oxygen in the 

 atmosphere falls below 13 per cent., the respiratory centre begins 

 to be excited by want of oxygen. It is probable that other fatigue 

 products, such as sarcolactic acid, assist the carbon dioxide in 

 stimulating the respiratory centre. 



In connection with the relative importance of the nervous and 

 chemical factors in breathing, F. H. Scott has shown that the 

 principal respiratory nerves (the pneumogastrics) regulate the 

 rate or rhythm of the respiratory movements, whilst the chemical 

 factor specially regulates the amount of pulmonary ventilation, that 

 is, the depth of the individual respiratory efforts; for when these 

 nerves are divided, a rise in the alveolar tension of carbon dioxide 

 (or great diminution of the oxygen in the respired air) increases the 

 depth, but not the rate of breathing. 



To recapitulate : In a normal respiration the chemical and 

 nervous factors would, therefore, appear to be related somewhat as 

 follows : The inspiratory centre makes an effort, the degree of 

 exaltation of the centre, and therefore, the magnitude of the effort, 

 more especially in the matter of depth, is governed by the tension of 

 carbonic acid in the blood, but it is cut short by an inhibitory 

 impulse passing up the vagus, only to begin again when the effects 

 of this inhibitory impulse are removed. 



During foetal life the need of the embryo for oxygen is small, and is amply met 

 by the transference of oxygen from the maternal blood through the thin walls of 

 the foetal capillaries in the placenta. But when the child is born, this source of 

 oxygen is no longer available, and the increasing venosity of the blood stimulates 

 the respiratory centre to action, and is the essential cause of the first inspiratory 

 eiforts the new-born child makes to obtain the oxygen it requires. It is said that 

 if the placental circulation is stopped while the child is still in utero, respiratory 

 eiforts are also made. Some regard the action of the air on the body surface as an 

 accessory cause of the first respirations, and it is the practice to increase this in 

 feeble children by stimulating the cutaneous nerves by the application of cold 

 water to the skin. Such treatment always causes deep inspirations, even in the 

 adult. There are other nerves stimulation of which influences the respiratory 

 act ; for instance, stimulation of the central end of the glossopharyngeal inhibits 

 the respiratory movements for a short period ; this accounts for the very necessary 

 cessation of breathing during swallowing. Stimulation of the central end of the cut 

 superior laryngeal nerve, or of its terminations in the mucous membrane of the 

 larynx, as when a crumb is " swallowed the wrong way," produces an increase of 

 expiratory efforts, culminating in coughing. 



