1096 



PHYSIOLOGY 



abolished by destruction of this centre. In this region we also find the 

 vaso-motor centre, so that the activity of one can affect that of the 

 other. This cardiac centre may be played upon by impulses arriving at 

 it through various afferent nerves or from the higher parts of the brain 

 and giving rise to the changes of the pulse-rate associated with the emo- 

 tional conditions, or it may be directly affected by the composition of 

 the blood circulating through its capillaries. 



The nerve-cells which give off the accelerator or augmentor fibres 

 are situated in the intermedio-lateral tract of the spinal cord, near 

 the point of origin of these fibres. We might therefore speak of an 

 augmentor centre in this region ; but it seems probable that the 



Sup-, lar. n. 



Depressor 



1-S.C.G. 



Symp. 



,--Vaqus 



Vagus-- 



Sup-, lar. n.-^ 



--- Sup-. Cerv. Gang. 

 Depressor 

 -Cerv. symp. n. 



Vaqo. syrnp. 



RABBIT 



DOG 



FIG. 453. Diagrams of the connections of the depressor nerve in the rabbit and 

 dog, according to Cyon. It will be noticed that in the latter animal the 

 depressor nerve runs in the vagus trunk for the greater'part of its course. 



activity of these cells is subordinate to impulses arriving at them from 

 the common meeting-place of visceral impulses, viz. the medulla. 



The most important of the afferent nerves which affect reflexly the 

 action of the heart are the nerves coming from the heart itself and the 

 aorta. In the mammalian ventricle nerve fibres can be seen running 

 over the surface of the ventricle which are entirely afferent, stimulation 

 of their peripheral ends causing no effect on the heart-beat. Stimula- 

 tion of their central ends may cause one of four conditions : 



(a) Slowing of the heart. 



(b) Rise of blood pressure from constriction of the splanchnic 

 area. 



(c) Fall of blood pressure by dilatation of the arterioles of the 

 body. 



(d) Reflex movements. The heart does not seem to be provided 



