272 COMPARATIVE PHYSIOLOaY. 



beat ; the sympathetic accelerate it or augment its force. When 

 both are stimulated together, the inhibitory prevail. 



These nerves, as also the accelerators, exercise a profound 

 influence upon the nutrition of the heart, and affect its electri- 

 cal condition when stimulated, and we may believe when influ- 

 enced by their own centers. 



The inhibitory fibers tend to preserve and restore cardiac 

 energy ; the sympathetic, whether in the vagus or as the aug- 

 mentors, the reverse. The vagus nerve (and probably the de- 

 pressor) acts as an afferent, cardiac sensory nerve reporting on 

 the intra-cardiac pressure, etc., and so enabling the vaso- 

 motor and cardio-inhibitory centers, which are, it would seem, 

 capable of related and harmonious action to act for the general 

 good. 



The arterioles must be conceived as undergoing very fre- 

 quent changes of caliber. They are governed by the vaso-motor 

 center, situated in the medulla, and possibly certain subordi- 

 nate centers in the spinal cord, through vaso-motor nerves. 

 These are (a) vaso-constrictors, which maintain a constant but 

 variable degree of contraction of the muscle-cells of the vessels ; 

 (6) vaso-dilators, which are not in constant functional activity ; 

 and (c) mixed nerves, with both kinds. An inherited tendency 

 to rhythmical contraction throughout the entire vascular sys- 

 tem, including the vessels, must be taken into account. 



The depressor nerve acts by lessening the tonic contraction 

 of (dilating) the vessels of the splanchnic area especially. 



It is important to remember that all the changes of the 

 vascular system, so long as the nervous system is intact — i. e., 

 so long as an animal is normal — are correlated ; and that the 

 action of such nerves as the depressor is to be taken rather as 

 an example of how some of these changes are brought about, 

 mere chapters in an incomplete but voluminous history, if we 

 could but write it all. The changes in blood-pressure, by the 

 addition or removal of a considerable quantity of blood, are 

 slight, owing to the sort of adaptation referred to above, effected 

 through the nervous system. Finally, the capillary circulation, 

 when studied microscopically, and especially in disordered con- 

 ditions, shows clearly that the vital properties of these vessels 

 have an important share in determining the character of the 

 circulation in themselves directly and elsewhere indirectly. 



The study of the circulation in other groups shows that 

 below birds the arterial and venous blood undergoes mixture 



