CIRCULATION IN BLOOD-VESSELS 47 



connecting with the ventricles and carrying blood away from 

 the heart are called arteries and the pressure in these is high, but 

 lower than in the heart. Those vessels connecting with the auri- 

 cles and carrying blood back to the heart are called veins and 

 the pressure is lowest of all in these. 



The minute vessels that connect the arteries and veins and 

 collect waste from and supply nutritive material to the lymph 

 stream are called capillaries. The 

 pressure in these is lower than in the 

 arteries but higher than in the veins. 



The blood is thus kept in motion, 

 constantly going from place of higher 

 to lower pressure. FIG. 23. Tracing of blood 



The completed circulation is thus: pressure taken with Prick's 



(Beginning with the right auricle of manometer - ( Y ^ 

 the heart.) The two venae cavse pour venous blood into the right 

 auricle and it in turn empties its contents into the right ventricle. 

 From here the blood is driven into the pulmonary artery (carrying 

 venous blood) to be aerated in the lungs. From the lungs it comes 

 by pulmonary veins (carrying arterial blood) to the left auricle. 

 This is the lesser or pulmonary circulation. 



From the left auricle the blood goes into the left ventricle and 

 from here it is forced into the aorta and thus into the systemic 

 arteries, then through the capillaries to the veins and back by 

 means of the venae cavae into the right auricle. 



The complete cycle in man takes about twenty-two seconds. 



STRUCTURE OF THE BLOOD-VESSELS. 



Arteries. The arteries have three coats: (i) the external coat 

 called the tunica adventitia, which is composed of fibrous 

 tissue with a little plain muscular tissue; (2) middle coat or 



