376 RESPIRATION [CH. XXVI. 



vagus. The vagus is chiefly an afferent nerve in relation to respira- 

 tion. 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 coincides in position 

 with the sensory centre of the vagus. The existence of subsidiary 

 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 by such nerves as the vagus, but also by those from the 

 cerebrum; so that we have a limited amount of voluntary control 

 over the respiratory movements. 



The respiratory centre is probably twofold, consisting of an 

 inspiratory and an expiratory centre. Of these two the inspiratory 

 centre is so much the more active that its importance 1 is a subject of 

 universal agreement ; whereas, the existence of an expiratory centre 

 is doubted by some physiologists, who regard expiration as a mere 

 cessation of the active process of inspiration, and a mechanical falling 

 back of the tissues into their places. 



2. The Nervous Factor in Respiration. 



During normal respiration, as opposed to forced respiration, an 

 impulse passes from the lung to the respiratory centre during each 

 complete respiration. This has been discovered by placing the 

 vagus on non-polarisable electrodes connected to a galvanometer, 

 and observing the current of action which accompanies each impulse. 

 The action-current takes place at the height of each inspiration. 



The currents that occur in the vagus during respiration can be 

 investigated with the capillary electrometer, as was done by Alcock 

 and Seemann ; they can still be more accurately studied by the use of 

 Einthoven's string galvanometer (see p. 121). The accompanying 

 figures (fig. 300) are reproduced from Einthoven's work on the sub- 

 ject. They were obtained from a dog. 



In fig. 300 A, normal respiration was taking place, and the line E 

 is a tracing of the respiratory movements ; the lowermost line (H) is 

 a tracing of the heart-beats. The top tracing (E) is a photographic 

 record of the movement of the quartz fibre in the galvanometer,- 

 which was connected by electrodes to the vagus nerve. The vago- 

 electrogram, as we may term it, shows large waves, which indicate the 

 changes in the activity of the nerve in reference to respiration ; the 



