vi LYMPHATIC SYSTEM 97 



by the arteries, and from which it is received into the 

 veins. 



Thus by means of the microscope we are able to take 

 the final step in demonstrating the circulation. The 

 fact that the blood can flow in one direction only is 

 proved by the disposition of the valves of the heart and 

 of the veins, but the passage of the blood from the 

 smallest arteries to the smallest vein by a connecting 

 system of minute tubes or capillaries can be proved only 

 by the employment of considerable magnifying powers. 

 We see that the vascular system of the frog is a closed 

 system of vessels : the blood is everywhere confined 

 within definite tubes through which it flows in a definite 

 direction, never escaping, as in some of the lower 

 animals, into large irregular spaces among the tissues. 



The Lymphatic System. Included in the vascular 

 system are certain cavities and vessels containing lymph, 

 and together constituting the lymphatic system. We have 

 already noticed the subcutaneous lymph sinuses (p. 18, 

 and Fig. 23, s.c.ly.s) and the sub-vertebral lymph-sinus 

 (p. 27, Fig. 5, s.v.ly.s). There are also found in nearly 

 all parts of the body delicate, thin- walled, branching 

 tubes, the lymphatic vessels (Fig. 23, ly. v). Unlike the 

 blood-vessels, the lymphatics are all of one kind, there 

 being no distinction into anything of the nature of 

 arteries and veins. They arise in lymph-capillaries (ly. 

 cp), which are, as it were, interwoven with the blood- 

 capillaries, but have no connection with them. By the 

 lymph-capillaries the fluid which has exuded from the 

 blood in its passage through the tissues is taken up and 

 passed into the lymphatic vessels or sinuses, and these 

 in their turn finally communicate with certain trans- 

 parent muscular organs called lymph-hearts. Of these 

 there are two pairs. The anterior lymph-hearts (a. ly. 

 ht) lie, one on either side, beneath the supra-scapula 



TRACT. ZOOL. 



