524 



Journal of Agriculture. 



[lo Sept., 1908. 



is forced into the pulmonary artery ; the ventricle stops ; the semilunar 

 valves close; and the blood in the pulmonary artery must perforce pass 

 onwards into a series of branches of which the first two into which the 

 artery divides are destined one for each lung. In the same manner as 

 already described a network of capillaries is formed which in this case 

 lines the air cells of the lung. Here a marked change is produced. 

 The blood which has entered the right auricle and ventricle and from 

 this has gone into the pulmonary artery, is venous ; here in the lung 

 capillaries it gives off some carbon dioxide and gains oxvgen so that it 

 changes its colour from purple to bright red. The capillaries pass inta 

 venules and then into veins. The flow of blood through the lung is 



Fig. 52. Diagram of Circulation. (After Halliburton.) 



sluggish, the pressure in the pulmonary arterv is small, and as the whole 

 system is inside the thorax the suction of the latter cannot count ; but 

 as the lung is continually shrinking and expanding the blood is worked 

 along, aided by back pressure, until it enters the great pulmonary veins 

 and then flows quietly into the left auricle. Thus the double circuit is 

 completed. 



The circulation from left ventricle to right auricle is called the 

 GREATER Or SYSTEMIC circulation ; that from right ventricle to left auricle 

 is called the lesser or pulmonary circulation. The shortest time the 

 blood can take to pass through both circuits is 31 seconds in the horse;: 

 this, it must be remembered, is the shortest, not the average time. 



A knowledge of this mechanism will explain the following peculiari- 

 ties of the circulation in, say, a limb — 



I. If a ligature be tied very tightly round the base of a limb, both 

 arteries and veins will be compressed, the blood in the limb will 

 be stagnant ; none will enter and none will leave. If the liga- 

 ture be tied less tightly so as to compress the veins and not 



