TYPICAL STRUCTURE OF ORGANS 



37 



We have seen (Chap. II, sect. 19) that each organ receives 

 blood through one or more arteries, and that this blood flows 

 away from the organ through one or more veins. If a colored 

 fluid mass which would afterwards set (for example, a warm 

 solution of gelatin colored with carmine) had been forced 

 into the arteries before we began our examination, we should 

 find that this mass would every- 

 where be confined in a system of 

 closed tubes which merely lie in 

 the connective tissue. The artery 

 entering the muscle branches into 

 smaller and smaller arteries in the 

 general sheath of the organ, or in 

 its branches, the septa; from these 

 finer arteries an exceedingly rich 

 network of small thin-walled tubes 

 is given off to the finest connective 

 tissue which surrounds the cells 

 themselves ; these tubes are the 

 capillaries. They ultimately unite 

 to form the larger veins, which can 

 be traced in the septa to those veins 

 which gross dissection reveals as 

 leaving the organ (see Fig. 19). 

 Through these tubes arteries, capil- 

 laries, and veins the blood flows; 

 and it is important for us to under- 



FIG. 28. Three muscle fibers 

 and an artery breaking up 

 into capillaries between them 



stand that it is everywhere confined to them in its passage 

 through the organs ; nowhere does it come into direct contact with 

 the living cells (save those lining the vessels). Whatever ex- 

 change of matter or energy takes place between the blood and 

 the living cells must be through the walls of the blood vessels. 1 



1 The term " blood vessel " is sometimes confusing to the beginner, since 

 it suggests a utensil for holding liquids. In anatomy "vessel" te a name 

 for tubes, ducts, or canals through which blood or lymph flows. 



