276 THE HUMAN BODY. 



of the divided cord causes widespread arterial constriction. 

 The main centre for the vaso-constrictors must then lie as 

 far forward as the medulla: and as all the brain in front of 

 the medulla oblongata can be removed without any con- 

 sequent arterial paralysis, the centre must lie in the medulla 

 itself. This centre is often named the vaso-motor centre, but 

 it is better to distinguish it as the vaso-constr ictor from the 

 centre for the dilator efferent nerves. 



The Control of the Vaso-constrictor Centre. The vaso- 

 constrictor centre is automatic; it maintains a certain amount 

 of activity of its own, independently of any stimuli reaching 

 it through afferent nerve-fibres. Nevertheless, like nearly all 

 automatic nerve-centres, it is under reflex control, so that its 

 activity may be increased or lessened by afferent impulses 

 conveyed to it. Nearly every sensory nerve of the Body is in 

 connection with it; any stimulus giving rise to pain, for 

 example, excites it, and thus constricting the arteries, in- 

 creases the peripheral resistance to the blood-flow and raises 

 arterial pressure. On the other hand, certain fibres conveying 

 impulses from the heart inhibit the centre and dilate the 

 arteries, lower blood-pressure, and diminish the resistance to 

 be overcome by the heart. These afferent fibres, which have 

 been already referred to as the large medullated fibres (p. 263) 

 of the pneumogastric, are known as the depressor fibres, or in 

 certain animals, for example the rabbit, where they are all 

 collected into one branch, as the depressor nerve. If this 

 nerve be divided and its cardiac end stimulated no effect is 

 produced, but if its central end (that still connected with the 

 rest of the pneumogastric trunk and through it with the 

 medulla oblongata) be stimulated, arterial pressure gradually 

 falls; this result being dependent upon a dilatation of the 

 small arteries, and consequent diminution of the peripheral 

 resistance, following an inhibition of the vaso-constrictor 

 centre brought about by the depressor nerve. Through the de- 

 pressor nerve the heart can therefore influence the calibre of 

 the small arteries and, by lowering aortic pressure, diminish its 

 own work if need be. In Fig. 103 is reproduced a tracing of 

 the great but slow fall of blood-pressure which results from 

 stimulation of the depressor fibres. It shows the slow fall of 

 pressure and slightly changed pulse-rate accompanying the 

 slow dilatation of the arteries, and may be compared with the 

 rapid fall and slow pulse brought about (Fig. 102) by excita- 





