554 THE OUTGO OF ENERGY 



XIII 



THE INNERVATION OF THE HEART AND 

 BLOOD-VESSELS 



THE quantity of blood required by the tissues 

 varies from time to time. For example, the 

 digestive organs require more blood when food 

 is taken than at other times. Variations in the 

 blood supply of the individual organs are accom- 

 plished chiefly by varying the size of their blood 

 vessels. To this end the blood vessels are pro- 

 vided with muscular coats which are made to 

 contract or relax, and thus to constrict or dilate 

 the vessels. The impulse to contraction or relax* 

 ation is given by the vasomotor nerves. It is 

 necessary, too, that the force and frequency of 

 ventricular contraction should vary with the 

 resistance to be overcome, the need for more 

 rapid oxygenation of the blood, etc., and special 

 nerves are provided for this purpose also. The 

 control or innervation of the heart and blood 

 vessels will now be considered. 



The heart is provided with nerves that aug- 

 ment and nerves that inhibit its action. 



