CHAP, iv.] THE VASCULAR MECHANISM. 351 



increased flow of blood through the kidney, may make its appear- 

 ance ; but in this case, as we shall see later on in dealing with the 

 kidney, the flow of blood through the kidney may be increased 

 in spite of constriction of the rest of the splanchnic area, and, 

 besides, such an initial increase of urine speedily gives way to a 

 decrease. 



194. The effect of food on the vascular mechanism affords a 

 marked contrast to the effect of bodily labour. The most marked 

 result is a widening of the whole abdominal splanchnic area, accom- 

 panied by so much constriction of the cutaneous vascular area, 

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

 lize the tendency of the widening of the splanchnic area to lower 

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

 mean pressure. 



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, you'll 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 inhibiting the tonic constrictor 

 influences exerted along the abdominal 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 blood-pressure which is too high for its powers, the 

 condition of the heart starts impulses which, passing along the 

 depressor fibres up to the medulla oblongata, temper down so to 

 speak the blood-pressure to suit the cardiac strength. 



We have moreover reason to think that not only does the 

 heart thus regulate the blood-pressure by means of the depressor 

 fibres, but also that the blood-pressure, acting as it were in the 

 reverse direction regulates the heart-beat ; a too high pressure, by 

 acting directly on the cardio-inhibitory centre in the medulla 

 oblongata (either directly, that is as the result of the vascular 



