276 THE COMPLETE GRAZIER. BOOK n, 



of the lungs is collected by the pulmonary veins and poured by them 

 into the left side of the heart, whence it is, in its oxygenised state, 

 d riven out through a strong artery, the aorta, which through its 

 branches supplies the capillaries of all parts of the body except the 

 lungs. One of its branches, for example, supplies the kidney, where 

 the nitrogenous waste of the blood is separated in the urine, another 

 supplies the mammary gland in the cells of which the milk is elaborated 

 from the blood, various other ones break up into the innumerable 

 capillaries beneath the skin which permit of their blood giving up a 

 considerable quantity of moisture in the form of perspiration. But 

 sooner or later all the capillaries of the body in general pour their 

 blood into small veins, and these into larger veins, till finally all the 

 blood so collected is poured into one or other of the great veins, 

 the posterior vena cava and the anterior vena cava, which have already 

 been spoken of, and from them the blood passes again into the right side 

 of the heart. 



The walls of the blood capillaries are so thin as to allow of their 

 being permeated. Hence, it is in the vicinity of the capillaries that the 

 nutritive work of the blood is performed, and new tissue is built up, 

 and it is also by means of the capillaries that the waste materials that 

 accumulate in the tissues of the body find a road into the blood and are 

 carried away ; were they allowed to accumulate in the tissues disease 

 would speedily ensue. It is in these capillaries of the system in general 

 that the blood, in consequence of the duties it discharges, becomes 

 more laden with carbonic acid and changes colour from scarlet to black. 

 Moreover, in the irrigation of the tissues, which is a necessary conse- 

 quence of the oozing of the blood through the capillary walls for the 

 performance of its nutritive work, some means are requisite to convey 

 away the fluid which would otherwise accumulate, and these means are 

 afforded by the minute tubes called the lymphatic capillaries which 

 convey their watery contents, the lymph, 1 mostly into the thoracic duct, 

 so that in the end the overflow from the blood capillaries finds its way 

 again into the blood into that, in fact, which enters the right side of 

 the heart. 



As the mammary gland can only elaborate milk out of the materials 

 brought to it by the blood, it may be as well to mention the route which 

 the blood takes in travelling from the heart to the udder. The arterial 

 blood is pumped from the left side of the heart into the aorta, passing 

 along which, the blood reaches the external iliac artery, and this is 

 continued on into the femoral artery, extending more or less parallel 

 to the femur, or thigh-bone. The femoral gives off a branch, the pre- 

 pubic, which in turn gives off a branch, the external pudic, and this, 

 after passing through the inguinal ring, divides into two branches, the 

 anterior, or subcutaneous abdominal artery, and the posterior 

 abdominal, or mammary artery, and it is from these that the blood 

 supply of the capillaries of the mammary gland is immediately derived ; 

 of the two the mammary artery is the more voluminous. 



The blood after passing through the capillaries of the mammary 



Lat. lympha, water. 



