THE ANIMAL BODY. 29 
The blood is the source of all the nourishment of the organ- 
ism, including its oxygen supply, and is carried to every part of 
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 
plood 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 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 fac- 
tories (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be added to it and the injurious components withdrawn.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 
carried on. 
The food is in the mouth submitted to the action of a series 
of cutting and grinding organs worked by powerful muscles; 
