270 COMPARATIVE PHYSIOLOi&Y. 



vessels against peripheral resistance, in consequence of which 

 the arteries are always kept more than full (distended), the 

 flow through the capillaries and veins is constant — a very great 

 advantage, enabling the capillaries to accomplish their work of 

 feeding the ever-hungry tissues. While physical forces play a 

 very prominent part in the circulation of the blood, vital ones 

 must not be ignored. They lie at the foundation of the whole, 

 here as elsewhere, and must be taken into the account in every 

 explanation. 



As a consequence of the anatomical, physical, and vital char- 

 acters of the circulatory system, it follows that the velocity of 

 the blood is greatest in the arteries, least in the capillaries, and 

 intermediate in the veins. 



The veins with their valves, their superficial position and 

 thinner walls, make up a set of conditions favoring the onflow 

 of the blood, especially under muscular exercise. 



In the mammal the circulatory system, by reason of its con- 

 nections with the digestive, respiratory, and lymphatic systems, 

 and in a lesser degree with all parts of the body, especially the 

 glandular organs, maintains at once the usefulness and the fit- 

 ness of the blood. 



The arterioles, by virtue of their highly developed muscular 

 coat, are enabled to regulate the blood-supply to every part, in 

 obedience to the nervous system. 



The blood exercises a certain pressure on the walls of all 

 parts of the vascular system, which is greatest in the heart it- 

 self, high in the arteries, lower in the capillaries, and lowest in 

 the veins, in the largest of which it may be less than the atmos- 

 pheric pressure, or negative. The heart in the mammal consists 

 of four perfectly separated chambers, each upper and each 

 lower pa,ir working synchronously, intermixture of arterial 

 and Venous blood being prevented by septa and interference in 

 working by valves. The heart is a force-pump chiefly, but, to 

 some extent, a suction-pump also, though its power as such 

 purely from its own action and independent of the respiratory 

 movements of the chest is slight under ordinary circunistances. 

 In consequence of the lesser resistance in the pulmonary divis- 

 ion of the circulation, the blood-pressure within the heart is 

 much less in the right than in the left ventricle — ^a fact in har- 

 mony with and causative of the greater thickness of the walls 

 of the latter; for in the foetus, in which the conditions are dif- 

 ferent, this distinction does not hold. 



