CHAP, iv.] THE VASCULAR MECHANISM. 375 



parried by so much constriction of the cutaneous vascular area 

 and so much increase of the heart's beat as are sufficient to neutra- 

 lize the tendency of the widening of the abdominal vascular area 

 to lower the mean pressure, or perhaps even sufficient to raise 

 slightly the mean pressure. 



The widening of the abdominal vascular area, as we have 

 seen ( 179), is probably an inhibition of tonic vaso-constrictor 

 impulses as a reflex act, assisted possibly by some local action 

 due to the presence of the food and similar to that supposed to 

 take place in the skeletal muscles during contraction. We 

 have at present no clear evidence that the absorbed products 

 of digestion play any important part in this splanchnic dilation by 

 acting on the central nervous system ; but the concomitant 

 increase of the heart beat is probably due to this cause. We have 

 no exact knowledge of how the absorbed products bring this 

 about, and possibly the mode of action differs with the different 

 constituents of food. With regard to alcohol, which is so often 

 part of a meal, we may perhaps say that the character of its 

 effects, the quickening and strengthening of the beats, seems to 

 point to its setting in action the augmentor mechanism, but it 

 also probably acts directly on the cardiac tissues. In any case the 

 effects depend largely on the dose, and if this is large the direct 

 effects become prominent, and the ultimate result is a deleterious 

 weakening. 



Any large widening of the cutaneous area, especially if accom- 

 panied by muscular labour and the incident widening of the 

 arteries of the muscles, would tend so to lower the general blood 

 pressure (unless met by a wasteful use of cardiac energy) as 

 injuriously to lessen the flow through the active digesting viscera. 

 A moderate constriction of the cutaneous vessels on the other hand, 

 by throwing more blood on the abdominal splanchnic area without 

 tasking the heart, is favourable to digestion, and is probably the 

 physiological explanation of the old saying, " If you eat till you're 

 cold, youll live to be old." 



In fact during life there seems to be a continual give-and-take 

 between the blood vessels of the somatic and those of the splanchnic 

 divisions of the body ; to fill the one the other is proportionately 

 emptied, and vice versa. 



195. We have seen ( 174) that certain afferent fibres of 

 the vagus, forming in the rabbit a separate nerve, the depressor 

 nerve, are associated with the vaso-constrictor nerves and the vaso- 

 motor centre in such a way that impulses passing centripetally 

 along them from the heart- lower the blood pressure by diminishing 

 the peripheral resistance, probably inhibitiDg the tonic constrictor 

 influences exerted along the splanchnic nerves, and so as it were 

 opening the splanchnic flood-gates. We do not possess much exact 

 information about the use of these afferent depressor fibres in the 

 living body, but probably when the heart is labouring against a 



