RESPIRATORY CENTRE. 83 



those respiratory movements which are dependent on the 

 facial nerve : destruction of the lower part only, arrests 

 the thoracic movements. (2.) The respiratory centre acts 

 automatically, i.e., is self-acting, but (3) its activity is affected 

 by the condition of the circulating blood, in such a way 

 that the more abundantly it is supplied with arterial blood 

 the less active are the respiratory movements, and the fewer 

 muscles take part in them. Accordingly saturation of the 

 haemoglobin of the blood with oxygen produces apncea, 

 i.e., suspension of respiratory effort, while defective arte- 

 rialization produces increased activity of both centres, i.e., 

 hyperpncea, which, if prolonged and excessive (dyspnoea) 

 results in exhaustion. (See Asphyxia, p. 90.) (4.) The 

 centre receives through the vagus trunk two sets of nerve 

 fibres which act upon it antagonistically to each other. Of 

 these, one set are chiefly contained in the superior laryngeal, 

 the other probably exclusively in nerves distributed to the 

 bronchial tubes and lungs, of which some of the fibres are 

 also inhibitory. The influence of these fibres may be under- 

 stood by supposing either that the respiratory centre consists 

 of two parts, of which one is inspiratory, the other expiratory, 

 and that the second of these acts antagonistically to the 

 first, i.e. exercises an inhibitory influence over it, or that 

 there is one centre of which the action is affected in oppo- 

 site ways, according as one or the other set of fibres is ex- 

 cited. (5.) The condition which produces hyperpncea is 

 not excess of CO 2 but defect of oxygen. (6.) The influence 

 of increase of the temperature of the blood on the respi- 

 ratory movements resembles that of defect of oxygen. 



Experimental Proofs. I. Destruction of the vagal tracts produces sudden 

 cessation of respiratory movements, without convulsion ; but severance of the 

 fascic. teretes above the striae stops only the respiratory movements of the 

 face, the laryngeal and thoracic movements continuing. 2. After section of 

 both vagi and of the cord above the third cervical vertebra, the animal dies of 

 asphyxia, but the respiratory movements of the facial muscles and those of 

 the sternomastoids continue. 3. Section of both vagi below the sup. laryng. 

 produces diminished frequency, increased amplitude and altered rhythm of 

 thoracic respiratory movements, with prolongation of each inspiratory act. 



G 2 



