1 68 THE CIRCULATION OF THE BLOOD AND LYMPH 



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. It is of interest in connection with the subject 

 of death during the administration of anaesthetics, that the afferent 

 vagus fibres coming from the alveoli of the lungs can be chemically 

 stimulated when irritant vapours, such as chloroform, hydrochloric 

 acid, ammonia, bromine, or formaldehyde are inhaled through a 

 tracheal cannula, causing reflex arrest of the heart and of the respira- 

 tory movements and a fall of blood-pressure through vaso-dilatation 

 (Brodie). At a certain stage in chloroform anaesthesia, before it 

 has become very deep, comparatively trifling causes may bring 

 about great and sudden changes in the pulse-rate, owing to the 

 abnormal mobility of the vagus centre (MacWilliam). 



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. 183), 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. 78), which sometimes develops itself with even 

 greater suddenness than when the accelerator nerves are directly 

 stimulated. Occasionally, however, the augmentation is abruptly 

 followed by a typical vagus action. Here the reflex inhibitory effect 

 seems to break in upon and cut short the reflex augmentor effect. 



