CHAP, iv.] THE VASCULAR MECHANISM. 229 



in the medulla oblongata, either positively by augmenting the 

 normal inhibitory influences and so slowing the heart, or negatively 

 by depressing those influences and so quickening the heart. 



c. By reflex stimulation of the same centre. Cases of exalta- 

 tion through reflex stimulation have already been quoted. In- 

 stances of depression leading to quickening of the heart's beat are 

 not so clear. The afferent impulses may be started in any part of 

 the body; but, as we have seen, there seems to be a special 

 connection between this centre and the alimentary canal. 



5. By the accelerator nerves. We have however, at present, 

 no very satisfactory evidence of the natural activity of these nerves. 



B. The Peripheral Resistance is affected 



1. By the vital i.e. the nutritive condition of the tissue of the 

 part. This is again influenced by 



a. The quality (and quantity ?) of the blood brought to it. 



b. Through the agency of the nervous system, as is seen in 

 cases of inflammation caused by nervous influences. 



. Both these points are very obscure. 



2. By the varying calibre (constriction, dilation) of the minute 

 arteries, brought about 



a. By the blood or other stimulus acting directly on the peri- 

 pheral vaso-motor mechanism. 



b. By the blood or other stimulus acting directly on the vaso- 

 motor centres in the central nervous system. 



c. By reflex stimulation of the vaso-motor centres. 



d. By the quantity of blood supplied to the vaso-motor centre, 

 this being in turn dependent on the blood-pressure in the arteries 

 supplying the centre. Thus a regulative mechanism is established 

 for cases when the quantity of blood, as distinguished from its 

 quality, is changed (see p. 225). 



Through these intricate ties it comes to pass that an event 

 which takes place in one part of the body is felt, to a greater or 

 less extent, by all parts. To take a simple instance : a change in 

 the condition of the skin at any one spot, such as that produced by 

 the application of cold or heat, may lead, 



a. By direct local action to a constriction or dilation of the 

 vessels of the part, giving rise to local pallor or suffusion. 



/5. By reflex action through the central nervous system, to an 

 increase of the same local effects, and in addition to a change in 



