CHAP, iv.] THE VASCULAR MECHANISM. 215 



sections of the so-called medullary vase-motor centre, comes about 

 because section of or injury to this region exercises a strong 

 inhibitory influence on all the vaso-motor centres situated in the 

 spinal cord below. Owing to the special function of the medulla 

 oblongata in carrying on the all-important work of respiration, a 

 mammal whose medulla has been divided cannot be kept alive for any 

 length of time. We cannot therefore put the matter to the simple 

 experimental test of extirpating the supposed medullary vaso-motor 

 centre and seeing what happens when the animal has completely 

 recovered from the effects of the operation : we have to be guided 

 in our decision by more or less indirect arguments. We must not 

 attempt to discuss the matter fully here, but may say that, after all 

 due weight has been attached to the play of inhibitory impulses, 

 there still remains a balance of evidence in favour of the view 

 that the. region of the medulla of which we are speaking does act 

 as a general vaso-motor centre. It is not however to be regarded 

 as the single vaso-motor centre, whither afferent impulses from all 

 parts of the body must always travel before they can start vaso- 

 motor impulses along this or that nerve. We are rather to suppose 

 that the spinal cord along its whole length, contains, interlaced 

 with the reflex and other mechanisms by which the skeletal muscles 

 are governed, vaso-motor centres and mechanisms of varied com- 

 plexity, the details of whose functions and topography have yet 

 largely to be worked out. As in the absence of the sinus venosus 

 the auricles and ventricle of the frog's heart may still continue to 

 beat, so in the absence of the medulla oblongata, these spinal vaso- 

 motor centres provide for the vascular emergencies which arise. 

 As however in the normal entire frog's heart, the sinus, so to speak, 

 gives the word and governs the work of the whole organ, so the 

 medullary vaso-motor centre rules and co-ordinates the lesser 

 centres of the cord, and through them presides over the chief 

 vascular areas of the body. It is possible moreover that the me- 

 dullary centre is specially connected with the splanchnic nerves and 

 thus with the capacious vascular area of the abdominal viscera, and 

 in consequence possesses an additional importance. By means of 

 these vaso-motor central mechanisms, by means of the head centre 

 in the medulla, and the subsidiary centres in the spinal cord, the 

 delicate machinery of the circulation, which determines the blood 

 supply, and so the activity of each tissue and organ, is able to 

 respond by narrowing or widening arteries to the ever-varying 

 demands and to meet by compensating changes the shocks and 

 strains of daily life. 



Vase-motor nerves of the Veins. Although the veins are 

 provided with muscular fibres, and are distinctly contractile and 

 although rhythmic variations of calibre due to contractions may 

 be seen in the great veins opening into the heart, in the veins of 



