634 MANUAL OF PHYSIOLOGY. 



and cardiac centres. From this the nerves pass into the cord to 

 the spinal roots, by which they reach the sympathetic. 



The vaso-motor centre exerts a tonic or continuing action on 

 the vessels, holding them in a state of partial constriction or tone. 

 In this it may possibly be said to have an automatic action. 

 Though tonic state of activity of the centre may be called auto- 

 matic, it is really under the control of many complex reflex in- 

 fluences, which constantly vary the general tone, or effect local 

 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 

 vaso-motor 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 re- 

 duce 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 vaso-motor 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 im- 

 pulses. The action of this centre is habitually that of a restrain- 

 ing agent lessening the rate of the heart's contractions, and is 

 hence called a tonic inhibitory centre (see page 274). The ac- 

 tivity 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. 



