306 THE CIRCULATION IN THE BLOOD-VESSELS. [CH. xxi. 



respiration was being carried on shows the three sets of waves, 

 first the oscillations due to the heart beats, next in size those 

 due to the respiratory movements, which in their turn are super- 

 posed on the prolonged Traube-Hering waves. The lower tracing 

 was taken immediately after the cessation of the artificial respira- 

 tion and shows only the heart beats and the Traube-Hering waves. 



The Vaso-motor centre may be excited reflexly. The 

 afferent impulses to the vaso-motor centre may be divided into 

 pressor and depressor. 



Most sensory nerves are pressor nerves. The sciatic or the vagus 

 nerves may be taken as instances; when they are divided and 

 their central ends stimulated, the result is a rise of blood-pressure 

 due to the stimulation of the vaso-motor centre, and a consequent 

 constriction of the arterioles all over the body, but especially 

 in the splanchnic area. Fig. 287 shows the result of such an 

 experiment. It is convenient in performing such an experiment 

 to administer curare as well as an anaesthetic to the animal, in 

 order to obviate reflex muscular struggles. 



Depressor nerve. In most animals the depressor fibres are 

 bound up in the trunk of the vagus, but in some, like the rabbit, 

 cat and horse, the nerve runs up as a separate branch from the 

 heart and joins the vagus or its superior laryngeal branch and 

 ultimately reaches the vaso-motor centre. When this nerve is 

 stimulated (the vagi having been previously divided to prevent 

 reflex inhibition of the heart), a marked fall of arterial blood-pres- 

 sure is produced (see fig. 288). Stimulation of this nerve affects 

 the vaso-motor centre in such a way that the normal constrictor 

 impulses that pass down the vaso-constrictoriien r es are inhibited. 

 The fall of pressure is very slight after section of the splanchnic 

 nerves, showing that the splanchnic area is the part of the body 

 most affected. The normal function of this nerve is to adapt the 

 heart's action to the peripheral resistance : if the constriction of 

 the arterioles is too high for the heart to overcome, an impulse by 

 this nerve to the vaso-motor centre produces reflexly a lessening 

 of the peripheral resistance. 



N.B. The term depressor should be carefully distinguished from inhibi- 

 tory ; stimulation of the peripheral end of the vagus produces a fall of 

 blood- pressure due to inhibition (slowing or stoppage) of the heart (see 

 figs. 281 and 282) ; stimulation of the central end of the depressor nerve 

 produces a lowering of blood-pressure for a different reason, namely a 

 reflex relaxation of the splanchnic arterioles. 



Experiments on Vaso-motor nerves. The experiments on 

 the vaso-motor nerves are similar to those performed on other 



