154 A MANUAL OF PHYSIOLOGY 



The suggestion is that the normal tone of the centre is largely 

 dependent upon reflex impulses. Be this as it may, we know 

 that the activity of the inhibitory centre is profoundly influenced 

 and that both in the direction of an increase and of a diminu- 

 tion by impulses that fall into it through afferent nerves and 

 by stimuli directly applied to it. And we may assume that the 

 same is true of the augmentor centre. The common statement 

 that stimulation of the central end of one vagus, the other being 

 intact, produces distinct inhibition does not hold for all mammals. 

 In dogs this is sometimes the case, but often (under anaesthesia, 

 at any rate) there is little or no inhibition, or even augmentation. 

 In etherized cats, on the other hand, some inhibition is always 

 seen. Of all the afferent fibres of the vagus, the pulmonary 

 fibres produce the most marked reflex inhibition. The cardiac 

 fibres are much less effective. 



These pulmonary nerves also influence the respiratory and 

 vaso-motor centres. The respiration is temporarily arrested, 

 and the blood-pressure falls through the dilatation of the small 

 arteries when they are excited. As will be again pointed out 

 in connection with the subject of death during the administration 

 of anaesthetics, the afferent vagus fibres coming from the alveoli 

 of the lungs can be chemically stimulated when irritant vapours, 

 such as chloroform or ammonia, are inhaled (p. 235). 



The depressor nerve, a branch of the vagus, which is easily 

 found in the rabbit as a slender nerve running close to the 

 sympathetic in the neck, and a little to its inner side, but in the 

 dog is usually blended with the vago-sympathetic, falls into the 

 same category with the vagus itself as regards its reflex action 

 on the heart, to which it bears an important relation. In all 

 mammals some of its fibres end in the wall of the aorta, but 

 some of them may run down over the heart to the ventricle. 

 Stimulation of its peripheral end has no effect, for the fibres in 

 it which influence the circulation are afferent, not efferent. 

 But excitation of its central .end causes a marked fall of blood- 

 pressure (p. 169), accompanied by, but not essentially due to, 

 a distinct slowing of the heart. If the animal is not anaesthetized, 

 there may be signs of pain, and for this reason the depressor 

 has sometimes been spoken of, somewhat loosely, as the sensory 

 nerve of the heart. The abdominal sympathetic (of the frog) 

 also contains afferent fibres, through which reflex inhibition of 

 the heart can be produced when they are excited mechanically 

 by a rapid succession of light strokes on the abdomen with the 

 handle of a scalpel. 



On the other hand, when the central end of an ordinary 

 peripheral nerve like the sciatic or brachial is excited the common 

 effect is pure augmentation (Fig. 66), which sometimes develops 



