CHAP, iv.] THE VASCULAR MECHANISM. 343 



bulb we infer that these tonic impulses proceed from the spinal 

 bulb. 



On the other hand we may remove the whole of the brain 

 right down to the upper limits of the spinal bulb, and yet produce 

 no flushing, or only a slight transient flushing, of any part of the / 

 body and no fall at all, or only a slight transient fall, of the \ 

 general blood pressure. We therefore seem justified in assuming 

 the existence in the spinal bulb of a nervous centre, which we S V 

 may speak of as a vaso-motor centre, or the bulbar vaso-motor 

 centre, from which proceed tonic vaso- constrictor impulses, or 

 which regulates the emission and distribution of such tonic vaso- 

 constrictor impulses or influences over various parts of the body. 



174. The existence of this vaso-motor centre may moreover 

 be shewn in another way. The extent or amount of the tonic 

 constrictor impulses proceeding from it may be increased or 

 diminished, the activity of the centre may be augmented or 

 inhibited, by impulses reaching it along various afferent nerves ; 

 and provided no marked changes in the heart beat take place at 

 the same time, a rise or fall of general blood pressure may be 

 taken as a token of an increase or decrease of the activity of the 

 centre. 



In the rabbit there is found in the neck, lying side by side 

 with the cervical sympathetic nerve and running for some distance 

 in company with it, a slender nerve which may be ultimately 

 traced down to the heart, and which if traced upwards is found to 

 come off somewhat high up from the vagus, by two or more roots, 

 one of which is generally a branch of the superior laryngeal nerve. 

 This nerve (the fibres constituting which are in the dog bound up 

 with the vagus, and do not form an independent nerve) appears 

 to be exclusively an afferent nerve; when after division of the 

 nerve the peripheral end, the end still in connection with the 

 heart, is stimulated no marked results follow. The beginnings of 

 the nerve in the heart are therefore quite different from the 

 endings of the inhibitory fibres of the vagus, or of the augmentor 

 fibres of the sympathetic system ; the nerve has nothing to do 

 with the nervous regulation of the heart treated of in Sec. 5. 

 If now, while the pressure in an artery such as the carotid is being 

 registered, the central end of the nerve (i.e. the one connected 

 with the brain) be stimulated with the interrupted current, a 

 gradual but marked fall of pressure (Fig. 75) in the carotid is 

 observed, lasting, when the period of stimulation is short, some 

 time after the removal of the stimulus. Since the beat of the 

 heart is not markedly changed, the fall of pressure must be due to 

 the diminution of peripheral resistance occasioned by the dilation 

 of some arteries. And it - is probable that the arteries thus 

 dilated are chiefly if not exclusively those arteries of the ab- 

 dominal viscera which are governed by the splanchnic nerves ; for 

 if these nerves are divided on both sides previous to the experi- 



