CHAP, iv.] THE VASCULAR MECHANISM. 323 



the lower limbs take origin in the central nervous system higher 

 up than the level of the 5th dorsal nerve. 



If the section of the spinal cord be made above the level of 

 the 2nd dorsal nerve, in addition to the above mentioned results, 

 the vessels of the head and face also become dilated ; but in 

 consequence of the fall of general blood-pressure just mentioned, 

 these vessels never become so full of blood, the loss of tone is not 

 so obvious in them as after simple division of the cervical sym- 

 pathetic, 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 peri- 

 pheral 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 ob- 

 longata, 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 fal^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 vaso-motor centre, or the medullary vaso-motor 

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

 regulates the emission and distribution of such tonic vaso-con- 

 strictor impulses or influences over various parts of the body. 



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

 over 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 in- 

 hibited, 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 



212 



