44 AMERICAN FARMER'S HORSE BOOK. 



Each artery has three distinct coats. The outer one is ot 

 a cellular structure, and is capable of great distension; that 

 upon the inside is a serous membrane, presenting internally 

 a smooth surface, which serves to diminish the friction of 

 the blood as it rushes on. The middle coat, largely com- 

 posed of muscular fibers, is highly elastic. In it resides the 

 power — without which circulation would be all but impossi- 

 ble — of equalizing the flow of blood, retarding its impetuous 

 velocity when near the heart, and accelerating it when con- 

 siderably removed from that great central force-pump of the 

 system. 



Although made up of the same number of coats, the walls 

 of the veins are much thinner and weaker. These vessels 

 are much more numerous than the arteries, notwithstanding 

 which their ramifications may be described, in general terms, 

 as corresponding with the latter. Their iuternal area is 

 nearly double that of the arteries. They are, of course, much 

 less directly afiected by the action of the heart. Hence, their 

 pulsations are scarcely perceptible, and the blood flows 

 through them more slowly. A feature peculiar to the veins 

 is the existence of valves, of various construction, which 

 prevent the blood from returning upon its course, and as- 

 sist in impelling it toward the heart. These are sometimes 

 single, at others double, and occasionally arranged in threes 

 and fours, around the interior of most, though not all, of the 

 large veins. 



Thus far we have considered only the general circulation. 

 When we come to the beautiful process known as the pul- 

 monary circulation, a portion of our description must be ex- 

 actly reversed — the pulmonary artery conveying the impure, 

 dark blood from the right ventricle of the heart to the lungs, 

 where it is oxygenized, or purified, and thence returns to the 

 left auricle of the heart, through the pulmonary vein, pos- 

 sessed of a scarlet brightness. Interesting phenomena occur 

 in connection with what physiologists term the portal circu- 

 lation (pertaining to the liver), but they are exceptional and 

 local. We must pass on to a description of the general plan 



