STRUCTURE OF THE BLOOD-VESSELS 47. 



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. 



The completed circulation is thus : 



(Beginning with the right auricle of the heart.) The two 

 venae cavae 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 pulmon- -pic. 23. Aor- 

 ary artery (carrying venous blood) to be tic regurgitation. 

 aerated in the lungs. From the lungs it (Greene.} 

 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 sec- 

 onds. 



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 tunica media, composed of yellow, elastic tissue, and 

 (3) the inner coat or tunica intima, composed of endothe- 

 lium. 



Veins. The veins also have three coats, the external, the 

 middle and internal, as the arteries ; but the middle coat is 

 composed chiefly of inelastic, fibrous tissue. Thus the veins 

 lack the elasticity and contractility given to the arteries by 

 the middle coat. 



