152 HAND-BOOK OF PHYSIOLOGY. 



arteries and is greatest at the time of auricular systole. There is no peri- 

 odic variation in venous pressure, as there is in the arterial, except in 

 the great veins. 



FIG. 137. Normal arterial tracing obtained with Tick's kymograph in the dog. (Burdon- 

 Sanderson.) 



Variations of Blood Pressure. Many circumstances cause con- 

 siderable variations in the amount of the blood-pressure. The following 

 are the chief: (1) Changes in the beat of the Heart; (2) Changes in the 

 Arteries and Capillaries; (3) 'Changes- due to Nerve Action; (4) Changes 

 in the Blood; (5) Respiratory Changes. 



1. Changes in the Beat of the Heart. The systole and diastole of the 

 muscular chambers. The arterial tension increases during systole and 

 diminishes during diastole. The greater the frequency, moreover, of the 

 heart's contractions, the greater is the blood-pressure, cceteris paribus; 

 although this effect is not constant, as it may be compensated for by the 

 delivery into the arteries at each beat of a comparatively small quantity 

 of blood. The greater the quantity of blood expelled from the heart at 

 each contraction the greater is the blood-pressure. 



The quantity and quality of the blood nourishing the heart's substance 

 through the coronary arteries must exercise also a very considerable 

 influence upon its action, and therefore upon the blood-pressure. 



2. Changes in the Arteries and Capillaries. Variations in the degree 

 of contraction of the smaller arteries modify the blood -pressure by favor- 

 ing or impeding the accumulation of blood in the arterial system which 

 follows every contraction of the heart; the contraction of the arterial 

 walls increasing the blood-pressure, while their relaxation lowers it. 



3. Changes due to Nerve Action. As with the heart, so with the 

 blood-vessels, the action of the nervous system is very important in rela- 

 tion to the blood-pressure; regulating, as it does, not only the force, fre- 

 quency, and length of the heart's systole, but also the condition of the 

 arteries, both through the central and peripheral vaso- motor centres. As 

 this subject has not yet been fully considered it will be as well to treat of 

 it here. 



It is upon the muscular coat of the arteries that the nervous system 

 exercises its influence; the elastic element possessing, as must be obvious, 

 rather physical than vital properties. The muscular tissue in the walls 

 of the vessels increases relatively to the other coats as the arteries grow 

 smaller, so that in the smallest arteries it is developed out of all propor- 



