244 THE VASCULAR MECHANISM. 



contrast to the effect of bodily labor. The most marked result is a widening 

 of the whole abdominal splanchnic area, accompanied by so much constric- 

 tion of the cutaneous vascular area, and so much increase of the heart's beat, 

 as is sufficient to neutralize 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 accompanied by 

 muscular labor 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 with- 

 out tasking the heart, is favorable to digestion, and is probably the physio- 

 logical 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 bloodvessels of the somatic and those of the splanchnic divisions of the 

 body ; to fill the one, the other is proportionately emptied, and vice versa. ^ 



181. We have seen ( 160) 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 vasomotor 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 laboring against the 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 condition of the medulla itself or indirectly, by impulses reaching 

 the medulla along afferent nerves from various parts of the body) may 

 send inhibitory impulses down the vagus, or so slacken or tone down the 

 heart-beats. 



In the following sections of this work we shall see repeated instances, 

 similar to or even more striking than the above, of the management of the 

 vascular mechanism by means of the nervous system, and we, therefore, need 

 dwell no longer on the subject. 



We may simply repeat that at the centre lies the cardiac muscular fibre r 

 and at the periphery the plain muscular fibre of the minute artery. On 

 these two elements the central nervous system, directed by this or that 

 impulse reaching it along afferent nerve fibres, or affected directly by this or 

 that influence, is during life continually playing, now augmenting, now 

 inhibiting, now the one, now the other, and so, by help of the elasticity of 

 the arteries and the mechanism of the valves, directing the blood-flow ac- 

 cording to the needs of the body. 



