VASOCONSTRICTOR NERVES 231 



When the rate of respiration is somewhat more frequent (cf. Fig. 96), the 

 influence of expiration is felt for the first during the following inspiration and 

 vice versa; the aortic pressure rises therefore during expiration and falls during 

 inspiration. 



In still more frequent and shallow breathing the variations of the flow to the 

 right heart are so slight that no respiratory variations of blood pressure appear 

 in the greater circulation. 



Besides these, certain nervous events exercise an unmistakable influence on 

 the respiratory variations of blood pressure in the aorta. During expiration the 

 pulse rate decreases in consequence of an automatic stimulation of the vagus 

 (depressor effect), and the vascular tonus increases as the result of an automatic 

 stimulation of the vasomotor nerves (pressor effect). These factors make them- 

 selves felt only with a rhythm which is not too rapid, and even then they may 

 not be able to alter the course of variations in the aortic pressure produced by 

 the mechanical factors already discussed. 



Still other more or less regular variations of pressure (Traube-Hering waves) 

 occur in the aortic system, which may run parallel with certain periodic varia- 

 tions in the frequency and depth of respiration extending over several respira- 

 tory cycles (Cheyne-Stokes breathing, Chapter IX) or may be entirely inde- 

 pendent of them. But further discussion of their nature would lead us too 

 far at this time. 



Artificial respiration gives in all respects just the reverse effects of natural 

 respiration. Thus with inflation of the lungs the blood flow to the right heart 

 is rendered more difficult on account of the positive intrapulmonary pressure, 

 and the resulting compression of pulmonary vessels. The consequence is that the 

 aortic pressure rises at the beginning of inflation, and falls again in the further 

 course of the same phase. 



The cause of these artificial pressure variations must be mainly the altera- 

 tions in diameter of the pulmonary vessels. At the beginning of inflation the 

 blood pressure rises because of the compression produced and the consequent 

 emptying of the blood toward the heart. The subsequent fall is the result of 

 the increased resistance in the pulmonary vessels. At the beginning of the arti- 

 ficial collapse a certain quantity of blood remains behind in the dilated vessels 

 and the pressure sinks still farther until the influence of diminished resistance 

 in the pulmonary vessels makes itself felt and the left heart is more abundantly 

 supplied. 



9. VASOCONSTRICTOR NERVES 



The muscular coat of the blood vessels is under the influence of two kinds 

 of nerve fibers, namely., those through whose excitation the muscle fibers are 

 caused to contract (vasoconstrictor nerves), and those through whose excita- 

 tion the muscle fibers are caused to relax (vasodilator nerves). The former 

 were discovered by Claude Bernard and Brown-Sequard (1851, 1852), the 

 latter by Schiff (1855) and Claude Bernard (1858). The importance of 

 the vasomotor nerves for the circulation was first clearly established by Lud- 

 wig (1864). 



If the cervical sympathetic be cut, one observes among other things that 

 the vessels of the ear dilate so that small arteries and veins which were for- 

 merly invisible now stand out clearly. If the edge of the ear be snipped off, 

 blood flows more freely from the wound than before section of the nerve. 

 The temperature of the ear is higher than that of the other side. Blood flows 

 16 



