CHAP, ii.] RESPIRATION. 629 



It is stated that after removal of the brain, the effect of stimu- 

 lating afferent fibres is more frequently and clearly inspiratory. 

 We may say, however, that in all cases the exact effect of any 

 stimulus applied to any afferent nerve is very largely determined 

 by the condition at the time being of the respiratory centre 

 itself ; and that is in turn determined not only by things which 

 affect its nutrition, such as the character of the blood circulating 

 in it, but also by the nature and amount of the other afferent 

 impulses which are playing upon it at the same time. Thus, as we 

 shall presently see, the effect of a stimulus applied to the vagus, 

 when the respiratory centre is inadequately supplied with arterial 

 blood, is more powerful than when the centre has its normal 

 supply of normal blood. So also a stimulus, which applied to 

 the vagus or to another nerve in an intact animal simply quickens 

 inspiration, applied in an animal whose cerebral hemispheres have 

 been removed will call forth a prolonged tetanic inspiratory gasp. 

 The respiratory centre responds in fact in the most intricate and 

 varied manner to nervous impulses proceeding from all parts of 

 the body, and thus delicately adjusts the working of the respiratory 

 pump to the needs of the economy. 



369. The complicated nature of the respiratory centre 

 is further shewn by the fact that it appears to consist of two 

 lateral halves which normally work in unison and yet may be 

 made to work independently. If the spinal bulb be carefully 

 divided in the middle line respiration may continue to go on in 

 quite a normal fashion. If, however, one vagus be then divided, the 

 respiratory movements, both costal and diaphragmatic, on the side 

 of the body on which division of the vagus has taken place, become 

 slower than those on the other side, so that the two sides are no 

 longer synchronous ; and a stimulus confined to one vagus affects 

 the respiratory movements of that side of the body only. So also 

 a section of a lateral half of the cord below the bulb stops the 

 respiratory movements on that side alone. 



370. Besides these nervous influences, however, there is 

 another circumstance which perhaps above all others affects the 

 respiratory centre, and that is the condition of the blood in respect 

 to its respiratory changes ; the more venous (less arterial) the 

 blood, the greater is the activity of the respiratory centre. When 

 by reason either of any hindrance to the entrance of air into the 

 chest, or other interference with the due interchange between the 

 blood and the pulmonary air or of a greater respiratory activity of 

 the tissues, as during muscular exertion, the blood becomes less 

 arterial, more venous, i.e. with a smaller charge of oxygen and more 

 heavily laden with carbonic acid, the respiration from being normal 

 becomes laboured. We may speak of normal breathing as eupnoea, 

 and say that this, when the blood is insufficiently arterialized, 

 passes into dyspnoea, an intermediate stage in which the respiratory 

 movements are simply exaggerated being known as hyperpnoea. 



