74 HUMAN PHYSIOLOGY 



variations in resistance, the decrease predominates in the 

 larger arteries, while the increase predominates in the veins, 

 so that in the larger arteries the fall is but little; in 

 the veins, large. In regard to the extent of the fall in 

 the smaller arteries and capillaries, the evidences are con- 

 flicting. 



5. Valves in the veins. The circulation of blood in the 

 veins is aided by externally compressing them, as occurs by 

 the contraction of surrounding muscles ; the regurgitation of 

 the blood is prevented by valves, somewhat like the semi- 

 lunar valves, which allow the blood to flow in the direction 

 toward the heart only. 

 6. Circulation time. 



To determine the time of a complete circulation ferrocyanide 

 of potassium is injected into the central end of a severed vein, the 

 time of injection being noticed. After some time the blood from 

 the peripheral end of the cut vein is tested for the salt by being- 

 colored blue with ferric chloride. The blood has completed the 

 entire circulation when the salt reappears at the peripheral end of 

 the vein. 



In dogs the circulation time has been found to be fifteen 

 seconds, in man it is supposed to be twenty-two seconds. 



4. IXXFRYATIOX OF THE OROAXS OF CIRCT LATK )X 



The influence of the central nervous system on the organs 

 of circulation (heart and muscles of vessels) serves to regu- 

 late the general velocity and distribution of the blood to the 

 different parts of the body. This is brought about by 

 changes in the number and strength of heart-beats and by 

 changes in the tonus of the muscles of the vessels, especially 

 of the arteries. 



i. Innervation of the heart (see also page 66). 



(a) The cardiac inhibitory nerves are the two vagi from 

 which fibres for the cardiac plexus are derived. Section of 

 the vagi results in increasing the pulse frequency. The 

 cardiac vagi are therefore continually stimulated (tonic). 

 Stimulation of the peripheral end of a cut vagus causes a 

 diminution in rate and strength of heart-beat or entire 



