REFLEX RESPIRATION. 335 



therefore, require at least 4 cubic 

 metres of pure air each hour we breathe. Taking into 

 account, however, the various combustions and decomposi- 

 tions taking place around us, and which contribute largely to 

 the vitiation of the air, hygienists have doubted the accuracy 

 of this figure, and it is generally admitted that, in order to 

 fulfil all the requirements of hygiene, a man needs 10 metres 

 of pure air every hour. 



V. INFLUENCE OF THE NERVOUS SYSTEM ON RESPIRATION 



1. The Respiratory Nervous Centre. The mechanical 

 phenomena of respiration (inspiration and expiration) are 

 reflex acts of which the nervous centre is found in the medulla 

 oblongata. (bulb), at the level of the gray matter of t-'ie fourth 

 ventricle, near the origin of the pneumo - gastric and the 

 spinal nerves. Galen pointed out the importance of this 

 point, and the sudden cessation of respiration (that is to say, 

 of life) which follows injury to the medulla oblongata; but 

 the investigations of Legallois and Flourens 1 have served to 

 decide the position of this point or nceud vital more ex- 

 actly. 



This centre is situated near those of the motor nerves of 

 the tongue (hypoglossal), of the lips (inferior ganglion of the 

 facial nerve), and of the cardiac fibres of the spinal and the 

 pneumo-gastric nerves. Labio-glosso-laryncjeal paralysis, 

 which has been so carefully studied by Duchenne (of Bou- 

 logne), results from attacks of these centres successively: the 

 tongue is generally affected first; some months later, the 

 muscles of the palate are attacked ; then the orbicular is' or is ; 

 followed by an attack of suffocation and by syncope. 2 



We have already seen that the blood may directly influ- 

 ence this respiratory centre, according as it abounds in 

 oxygen or in carbonic acid, and, especially, that an excess of 

 carbonic acid coming in contact with the gray matter (in the 

 4th ventricle) of this nervous centre, increases to the high- 

 est degree the desire to breathe. The first respiratory move- 

 ment of the foetus is, no doubt, caused by the sudden inter- 

 ruption of the placental respiration (see p. 324), producing 





1 See Flourens, " Recherches Experimentales sur le Systeme 

 Nerveux." 1842, p. 196. 



2 Duchenue (de Boulogne), " De 1' Electrisation Localisee." 

 1872, p. 5G4. 



