12 THE DOG IN HEALTH. 



tween the cells surrounding them and the blood which 

 may be compared to a process of barter, the cells taking 

 nutriment and oxygen and giving (excreting) in return 

 waste products. From these minute vessels the blood 

 is conveyed back toward the source whence it came by 

 similar elastic tubes, which gradually increase in size and 

 become fewer. The force w^hich directly propels the 

 blood in its onward course is a muscular pump (heart), 

 with both a forcing and suction action, though chiefly 

 the former. The flow of blood is maintained constant 

 owing to the resistance in the smaller tubes on the one 

 hand and the elastic recoil of the larger tubes on the 

 other ; while in the returning vessels the column of blood 

 is supported by elastic double gates (valves), which so 

 close as to prevent reflux. The oxygen of the blood is 

 carried in disks of microscopic size, which give it up in 

 proportion to the needs of the tissues past which they are 

 carried. 



But in reality the tissues of the body are not nourished 

 directly by the blood, but by a fluid derived from it and 

 resembling it greatly in most particulars. This fluid 

 bathes the tissue-cells on all sides. It also is taken up by 

 tubes that convey it into the blood after it has passed 

 through little factories (lymphatic glands), in which it un- 

 dergoes a regeneration. Since the tissues are impover- 

 ishing the blood by withdrawal of its constituents and 

 adding to it what is no longer useful and is in reality poi- 

 sonous, it becomes necessary that new material be added 

 to it and the injurious components withdrawn. The for- 

 mer is accomplished by the absorption of the products of 

 food digestion and the addition of a fresh supply of oxy- 



