224 THE VASCULAR MECHANISM. 



cervical sympathetic, since the latter operation produces little or no effect on 

 the general blood-pressure. 



Obviously, then, the tonic vaso-constrictor impulses, which passing to the 

 skin and viscera of the body maintain that tonic narrowing of so many small 

 arteries by which the general peripheral resistance, and so the general blood- 

 pressure, is maintained, proceed from some part of the central nervous 

 system higher up than the upper dorsal region of the spinal cord. And, 

 since exactly the same results follow upon section of the spinal cord in the 

 cervical region right up to the lower limit of the medulla oblongata, we infer 

 that these tonic impulses proceed from the medulla oblongata. 



On the other hand, we may remove the whole of the brain right down to 

 the upper parts of the medulla, 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 medulla oblongata of a nervous centre, 

 which we may speak of as a vasomotor centre, or the medullary vasomotor 

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

 lates the emission and distribution of such tonic vaso-constrictor impulses or 

 influences over various parts of the body. 



160. The existence of this vasomotor centre may, moreover, be shown 

 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 upward 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 splanchnic (sympathetic) system ; 

 the nerve has nothing to do with the nervous regulation of the heart (see 

 139 et seq.*). 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. 79) 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 dilatation 

 of some arteries. And it is probable that the arteries thus dilated are 

 chiefly, if not exclusively, those arteries of the abdominal viscera which are 

 governed by the abdominal splanchnic nerves ; for if these nerves are 

 divided on both sides previous to the experiment, the fall of pressure when 

 the nerve is stimulated is very small in fact, almost insignificant. The 

 inference we draw is as follows : The afferent impulses, passing upward along 

 the nerve in question, have so affected some part of the central nervous 

 system that the influences which, in a normal condition of things, passing 

 along the abdominal splanchnic nerves keep the minute arteries of the 



