THE VASCULAR MECHANISM. 219 



The force of the mammalian heart beat is also influenced by changes 

 in the reaction of the blood. If the tension of carbonic acid in the 

 blood is greatly increased, the heart relaxes more completely during 

 diastole and contracts less forcibly during systole, with the result that 

 its output is diminished. A decrease in the tension of carbonic acid, on 

 the contrary, leads to the accumulation of blood in the great veins, less 

 blood enters the heart during diastole, and the output of the heart is 

 diminished. 



Both in the frog and in the mammal, the rate of the heart is 

 increased by a rise and decreased by a fall in the temperature of the 

 fluid circulating through it. With this exception, changes in the 

 character of the blood have little or no effect on the rate of the 

 heart beat. 



SECTION V. 

 THE REGULATION OF THE VASCULAR MECHANISM. 



In order that the various tissues of the body may receive an 

 adequate supply of nutritive material and oxygen, it is essential that 

 the blood supply to the different organs should be varied in accordance 

 with their needs. This end is attained by means of the central nervous 

 system, which can modify the rate of the heart and the calibre of the 

 arterioles in response either (1) to external stimuli, or (2) to impulses 

 arising in the different parts of the body itself, or (3) to changes in the 

 character of the circulating blood. This latter factor also directly 

 influences the force of the heart beat and the calibre of the vessels. 



THE INNERVATION OF THE HEART. 



The nerves supplying the heart are (1) the vagus, and (2) branches 

 from the sympathetic system. The sympathetic fibres arise in the 

 frog from the white ramus of the third spinal nerve, and have their 

 cell station in the corresponding sympathetic ganglion. From the 

 ganglion they pass upwards to join the vagus close to its exit from the 

 skull ; the combined vagus and sympathetic fibres form a single nerve 

 on each side, the vago-sympathetic, which runs to the heart (fig. 82). 

 The fibres of the vagus nerve have their cell station in the ganglia of 

 the heart itself. 



In the mammal the vagus gives off branches in the thorax, 

 which ..run direct to the heart, in which their cell stations lie. The 

 sympathetic fibres leave the spinal cord by the second and third 

 thoracic white rami ; their cell stations are in the stellate ganglion, 

 from which post-ganglionic fibres run directly to the heart. 



