242 



THE CIRCULATION OF THE BLOOD 



stimulation. The volume of the hind limb of a curarized and vagotomized 

 rabbit increases when the central end of the cardiac depressor nerve is 

 stimulated. In order to determine whether this dilatation is due solely 

 to the removal of vasoconstrictor tone, the above experiment was repeated 

 on a rabbit in which the sympathetic chain had been cut below the level 

 of the second lumbar spinal roots. By such an operation all the vaso- 

 constrictor fibers to the vessels of the hind limb are severed, but the 

 vasodilator fibers, since they emanate through the sacral sensory roots, 

 are left intact. It was nevertheless found on stimulating the depressor 

 nerve that dilatation of the hind limb still occurred, thus indicating 



Fig. 75. Diagram showing the probable arrangements of the vasomotor reflexes. 



A. Muscle of arteriole. 



D. Vasodilator nerve fiber terminating on A and inhibiting its natural tonus, as indicated by - 

 sign. 



D, Vasoconstrictor fiber also ending in A, but exciting it (+). These two kinds of fiber arise 

 from the dilator center (DC) and the constrictor center (CC) respectively. 



F. Afferent depressor fiber, dividing into two branches, one of which (-) inhibits the con- 

 strictor center, while the other (+) excites the dilator center causing dilatation of the arteriole and 

 fall of blood pressure. 



R. Pressor fiber exciting CC and inhibiting DC, and therefore causing vasoconstriction and rise 

 of blood pressure. 



a, b, c, and d represent the synapses of the pressor and depressor branches with the efferent 

 neurons. (From Bayliss.) 



that stimulation through vasodilator fibers must have taken place. Con- 

 versely, in another experiment, instead of the sympathetic chain, the 

 spinal cord was cut below the level of the second lumbar segment, thus 



