CH. XXIV.] FORCE OF RESPIRATION. 357 



of air admitted to the lungs, and to each part of them, according 

 to the supply of blood : the muscular tissue also contracts upon 

 and gradually expels collections of mucus, which may have 

 accumulated within the tubes, and which cannot be ejected by 

 forced expiratory efforts, owing to collapse or other morbid 

 conditions of the portion of lung connected with the obstructed 

 tubes (Gairdner). 



The Nervous Mechanism of Bespiration. 



In the central nervous system there is a specialised small 

 district called the Respiratory centre. This gives out impulses 

 which travel down the spinal cord to the branches of the spinal 

 nerves that innervate the muscles of respiration. It also receives 

 various afferent fibres, the most important of which are contained 

 in the trunk of the vagus. The vagus is chiefly an afferent nerve 

 in relation to respiration. It, however, also is in a minor degree 

 efferent, for it supplies the muscular tissue of the lungs and 

 bronchial tubes, and exercises a trophic influence on the lung. 



The respiratory centre was discovered by Flourens; it is situated 

 at the tip of the calamus scriptorius, and almost exactly coincides 

 in position with the centre of the vagus. The existence of sub- 

 sidiary respiratory centres in the spinal cord has been mooted, 

 but the balance of experimental evidence is against their existence. 

 Flourens found that when the respiratory centre is destroyed, 

 respiration at once ceases, and the animal dies. He therefore 

 called it the " vital knot " (noeud vitale). 



The centre is affected not only by the afferent impulses which 

 reach it from the vagus, but also by those from the cerebrum; so 

 that we have a limited amount of voluntary control over tho 

 respiratory movement. 



The sensory nerves of the skin have also an effect. The action 

 of the cold air on the body of a new-born child is no doubt the 

 principal afferent cause of the first respirations. During foetal 

 life, the need of the embryo for oxygen is very 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. 

 The application of cold water to the skin always causes a deep 

 inspiration ; this is another instance of the reflex effect which 

 follows stimulation of the cutaneous nerves. Stinnilation of the 

 central end of the splanchnics causes expiration. Stimulation of 

 the central end of the glosso-pharyngeal causes an inhibition of the 

 respiratory movements for a short period ; this accounts for the 

 very necessary cessation of breathing during swallowing. Stimu- 

 lation of the central end of the cut superior laryngeal nerve, or 



