136 A MANUAL OF VETERINARY PHYSIOLOGY 



and the condition of the centre at the time of the experiment 

 appear to be important factors in determining the exact results 

 which will follow. But the interpretation of the results of tnese 

 experiments is to show that there are two kinds of fibres in the 

 vagus conveying impulses from the lungs to the medulla, and 

 affecting it in opposite ways : (i) Fibres carrying impulses which 

 cause inspiration and inhibit expiration ; and (2) fibres convey- 

 ing impulses which inhibit inspiration, and so cause expiration. 

 These sets of fibres are in alternate activity, and the cause of 

 their normal stimulation is believed to be the alternate distension 

 and collapse of the air vesicles. If, for example, air be pumped 

 into the lungs, expiration is excited, and if it be sucked out, 

 inspiration follows ; from which it is argued that an inspiration, 

 by distending the air vesicles, excites expiration, and the con- 

 traction of the air vesicles on expiration excites inspiration. It 

 can be shown experimentally that electrical changes in the 

 divided vagus indicate a marked current throughout each in- 

 spiration, and another of a different character during expiration. 

 If, however, the respiratory centre be regarded as primarily 

 automatic, the inspiratory fibres found in the vagus may be 

 looked upon as increasing the rate of respiration, the expiratory 

 as inhibiting or controlling inspiration, and thus producing ex- 

 piration. If this view be adopted, the act of inspiration pro- 

 ceeds from an automatic centre, which requires no other stimulus 

 than that which is generated within itself, while expiration 

 proceeds from the stimulation caused by distension of the air 

 vesicles. 



The nature of the internal stimulus which provokes the respira- 

 tory centre has for some time been the subject of controversy. 

 All were agreed that it lay in the blood gases, but whether this 

 was to be attributed to richness in carbon dioxide or poverty 

 in oxygen could not for some time be decided. It is now pretty 

 generally admitted that the richness in carbon dioxide is a more 

 potent stimulant than poverty in oxygen, and Haldane's re- 

 searches mentioned at p. 120 show the extraordinary delicacy of 

 the respiratory centre's response to minute increases in carbon 

 dioxide, a 02 per cent, increase leading to an increased pul- 

 monary ventilation of 100 per cent. 



During muscular work, especially that of a severe nature, there 

 may be such substances as lactic acid in the circulating blood, 

 which also act as a stimulant to the respiratory centre. 



Respirations are in^? eased in frequency as the result of 

 sensory stimulation — *Ufh t for instance, as occurs in painful 

 operations ; sensory stimulation of the abdominal wall may be 

 employed as a means of starting an inspiration in chloroform- 

 poisoning. 



