634 MANUAL OF PHYSIOLOGY. 



changes in the degree of constriction of this or that vascular 

 area. Among the most striking afferent regulating impulses are 

 those arriving from the heart, the digestive organs, and the skin. 

 In some animals a special nerve the depressor has been dis- 

 covered, which, passing from the heart to the medulla, keeps the 

 vasomotor centre informed as to the degree of tension, etc., of 

 the heart cavities. When the heart becomes overfull, impulses 

 pass from it and check the tonic power of the centre so as to 

 reduce the arterial pressure against which the ventricle has to 

 act. Electric stimulation of this nerve causes a remarkable fall 

 in the general blood pressure. The vasomotor centres regulate 

 the distribution of blood to the viscera and skin, according to 

 the condition of activity of these parts as described in another 

 chapter (XXXI). 



THE CARDIAC CENTRE. 



Although the heart beats with characteristic periodicity when 

 cut off from the nervous centres, its normal rhythm is under the 

 control of a group of nerve cells in the medulla, from which some 

 of the fibres of the vagus conduct special regulating impulses. 

 The action of this centre is habitually that of a restraining agent 

 lessening the rate of the heart's contractions, and is hence called 

 a tonic inhibitory centre (see page 280). The activity of the 

 centre is influenced by the condition of many distant parts, such 

 as the cortex of the brain, the abdominal viscera, etc., which 

 exert a kind of reflex action on the heart through this centre. 

 The degree of inhibitory power, as well as the share taken in 

 the action of the centre by automatism and reflection, differs in 

 different animals. 



In the medulla there also exist many other centres connected 

 with the organic functions. Among these the centres for swallow- 

 ing and vomiting may be mentioned. For further details on 

 this subject, the reader may consult the chapter on Digestion (see 

 page 121). 



