30 COMPARATIVE PHYSIOLOGY. 



the body through, elastic tubes which, continually branching 

 and becoming gradually smaller, terminate in vessels of hair- 

 like fineness in which the current is very slow — a condition per- 

 mitting that interchange between 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 carbonic anhydride. 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 be- 

 come fewer. The force which directly propels the blood in its 

 onward course is a muscular pump, 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 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 resem- 

 bling it g^-eatly in most particulars. This fluid bathes the tis- 

 sue-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 undergoes a regeneration. 

 Since the tissues are impoverishing the blood by withdrawal of 

 its constituents, and adding to it what is no longer useful, and 

 is in reality poisonous, it becomes necessary that new material 

 be added to it and the injurious components withdrawn. The 

 former is accomplished by the absorption of the products of 

 food digestion, and the addition of a fresh supply of oxygen 

 derived from, without, while the poisonous ingredients that 

 have found their way into the blood are got rid of through 

 processes that may be, in general, compared to those of a sew- 

 .age system of a very elaborate character. To explain this re- 

 generation of the blood in somewhat more detail, we must first 

 consider the fate of food from the time it enters the mouth till 

 it leaves the tract of the body in which its preparation is car- 

 ried on. 



The food is in the mouth submitted to the action of a series 

 of cutting and grinding organs worked by powerful muscles ; 

 mixed with a fluid which changes the starchy part of it into 



