VI LYMPHATIC SYSTEM 97 



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 con- 

 siderable magnifying powers. We see that the vascular system 

 of the frog is a closed system of vessels : the blood is every- 

 where 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, 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 transparent 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 scapula and just behind the transverse process of 

 the third vertebra : the posterior pair {p. ly. ht) are situated 

 one on each side of the posterior end of the urostyle. 

 These organs pulsate regularly, like miniature hearts, and 

 pump the lymph into the veins, the anterior pair communi- 



PRACT. ZOOL, H 



