THE PHYSIOLOGY OF AN ANIMAL 209 



which lie near them and with which they are connected. In this 

 way the lymph, which originally came from the blood plasma 

 by dialyzing through the walls of the capillaries, returns into the 

 blood; thus all the secreted products from the living cells pass 

 into the blood, either directly as in the case of gases, or indirectly 

 by passing first into the lymph and then emptying with the 

 lymph into the blood vessels. 



NOTE. A similar lymphatic system is found in all higher animals, but 

 its course is different from that in the frog. In man, for example, the 

 lymph rises by diffusion through the capillaries, and collects in lacunae 

 and lymph vessels in a similar manner. But there are no lymph hearts. 

 The lymph vessels unite to form quite large vessels, and all eventually 

 empty into the large veins in the neck. There are two chief trunks of these 

 vessels, one bringing the lymph from the upper parts of the body and 

 emptying into the right jugular vein, and the other, a much larger one, 

 bringing the lymph from the lower parts of the body and from the alimen- 

 tary canal and flowing up through the thorax, to empty finally in the left 

 jugular vein. This latter lymph vessel is called the thoracic duct. 



Respiration. The impure blood from the heart passes through 

 the pulmonary artery to the lungs (Fig. 92), a part of it going 

 into a small branch, the cutaneous, cu, which carries it to the 

 skin. The lungs are air sacs connected with the mouth. Just 

 back of the tongue we have already noticed the glottis, which 

 is a slit leading into a small cavity holding the vocal cords, 

 whose vibrations cause the various sounds produced by the 

 animal. This cavity is the larynx and it lies just under the 

 throat. At its inner end it opens at once into the lungs, since 

 the frog has no windpipe (trachea) such as is found in animals 

 with long necks, like man. The air enters the lungs through the 

 larynx and, filling them, comes in close contact with the blood, 

 which is distributed in finely divided capillaries in their walls. 

 The blood that goes to the skin through the cutaneous artery 

 is distributed in fine capillaries and brought into close con- 

 tact with the oxygen which is dissolved in the water in which 

 the animal lives. 



The haemoglobin, which gives the red color to the red cor- 



