The Frog 



57 



white+cytos=cell), in the blood. These are often able to force them- 

 selves through the walls of the capillaries and then wander about through 

 the tissues. In the liquid part of the blood there is still a third type 

 of tiny bodies which are called platelets. Of these little is known. 



Beside the arteries and veins there are also the lymph vessels in the 

 skin, intestine, and other parts of the body which belong to the circula- 

 tory system. The liquid part of the blood in which the corpuscles float 

 is known as plasma. When the blood passes into all parts of the body 

 to nourish it, some of this plasma finds it way through the little arterial 

 capillaries, bathing the intercapillary spaces. This plasma, which has 

 left the blood vessel proper to bathe the body tissues, is called lymph. 

 The lymph must be gathered again and made a part of the blood, so 

 various little lymph capillaries drain the body and pour the lymph back 

 into the veins. These lymphatic vessels are very delicate, and must be 

 prepared in a special way to be seen. The little open spaces where 

 lymph gathers and from which the lymphatics carry it to the veins, are 

 known as a lacunae. These lacunae also connect with large cavities in 

 the body. 



The lymph vessels in the intestine have a special name, being called 

 lacteals ( ). There are also lymphatic glands found 



lymph 



Lymph 



Fig. 11. Section Through a Lymph Heart. (After Weliky.) 



kl — Tube-like valve at entrance of vein into the heart. 



v — Vein. 



lymph — Lymph-heart. 



in connection with the lymph vessels. In the frog there are two pairs 

 of lymph hearts (Figs. 11 and 347) whose contraction propels the lymph 

 in its circulation. 



With this in mind, we take up the principal divisions of the circula- 

 tory system. 



PRINCIPAL DIVISIONS OF THE CIRCULATORY SYSTEM 



THE HEART 



There is a true heart consisting, however, of only three cavities 

 (Fig. 10), two thin-walled auricles, ( ) one on the 



