THE STRUCTURAL AND FUNCTIONAL SYSTEMS 61 



to the left auricle, and is again delivered to the left ventricle. The 

 separation of oxygenated and unoxj-genated blood is thus com- 

 plete, since the two circuits are distinct and there is no mingling 

 in a common ventricle. A blood corpuscle, for example, may pass 

 to one part of the body or to another, but it cannot make its 

 "next trip" to any region without first making the circuit of the 

 lungs. Hence, every drop of blood passes once through the lungs 

 and once through some other part of the body in completing its 

 circulation. 



Reference has been made to the lymph spaces that lie beneath 

 the skin of the frog and to the large lymph sinus that lies dorsal 

 to the kidneys (Fig. 19). These are part of an extensive system of 

 irregular vessels and spaces extending throughout the body and are 

 filled with a colorless fluid, the lymph, which is essentially the same 

 as blood minus the red blood cells, or corpuscles, that give the blood 

 its color. In the frog, the lymph is pumped into the blood by four 

 lymph hearts (Fig. 50, p. 90) which are connected with lymph spaces 

 and empty into four of the principal veins. The lymph originates 

 largely by diffusion from the blood capillaries in all parts of the 

 body and thus circulates as lymph in its own vessels and also as 

 part of the fluid portion of the blood. In man, the lymph vessels, or 

 lyrnphatics, consist principally of ves.sels rather than spaces, so that 

 a part of the body, such as the hand, has lymph vessels so extensive 

 that they resemble a second set of veins (Fig. 20, p. 41). These 

 communicate with the blood vessels by two large lymphatic trunks 

 which empty, by a larger connection on the left and a smaller on 

 the right side, into the great veins of the neck. There are no 

 structures in man comparable with the lymph hearts of the frog, 

 and no such lymph sinuses beneath the skin. 



The circulatory system, therefore, consists of a series of vessels 

 through which the blood is caused to circulate by the action of a 

 specially modified area, the heart. In the course of its circulation, 

 the blood is distributed to all parts of the body, including organs 

 like the lungs, kidneys, and digestive tract, where it gives off and 

 takes up the substances that it carries to and from the various 

 regions (cf. Fig. 50, p. 90). Consideration of the blood as a "com- 

 mon carrier" presents many interesting problems, some of which 

 will be examined in the next chapter; but these do not properly 

 belong with the present discussion, which is principally concerned 

 with the structure of the circulatory system. ^.^ 



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