THE FROG — A REPRESENTATIVE VERTEBRATE 411 



flow (Fig. 21.13). A thin-walled sinus venosus receives blood from the 

 posterior and anterior venae cavae, and passes it into the right atrium. 

 The right atrium receives blood low in oxygen content from the body, 

 and blood high in oxygen content from the skin and lining of the 

 mouth. The pulmonary veins bring additional oxygen-rich blood from 

 the lungs to the left atrium. Both atria lead into a single ventricle having 

 a thick, muscular wall. The ventricle forces the blood through a final 

 chamber, the conus arteriosus, and into each truncus arteriosus. A 

 peculiar spiral valve is found in the conus. 



Some mixing of blood from the two atria takes place in the ven- 

 tricles, but how much is uncertain. According to the classic view, a 

 slight difference in the time of entrance of the blood from the two atria, 

 the spongy ventricular wall, and the deflective effect of the spiral valve 

 in the conus result in most of the blood from the left atrium passing 

 into the carotid and aortic arches, while most of the blood from the 

 right atrium passes into the pulmocutaneous arch. More recent studies, 

 in which opaque materials were injected into the blood and photo- 

 graphed with x-rays, indicate that there generally is little separation of 

 the blood streams in the ventricle, but that sometimes the postulated 

 separation takes place. The blood in the right atrium is partially oxy- 

 genated, for some of it has returned from vessels in the skin. The 

 problem is simply to what extent this is mixed with blood from the lungs 

 containing even more oxygen. 



1 88. Excretory System 



The skin and the lungs remove some waste products of metabolism, 

 but the kidneys are the major excretory organs and remove most of the 

 nitrogenous wastes. They also help to maintain the constancy of the in- 

 ternal environment by removing from the blood substances in excess and 

 by conserving those in short supply. 



The frog's kidneys are a pair of elongate organs lying in the sub- 

 vertebral lymph sac dorsal to the pleuroperitoneal cavity (Figs. 21.8, 

 21.14, 21.15). They are composed of a great many microscopic kidney 

 tubules that are intimately related to blood entering the kidneys in the 

 renal arteries and renal portal veins. These tubules are described more 

 fully in Chapter 28; briefly, they remove certain products from the blood 

 and carry them as urine to the Wolffian ducts. A Wolffian duct, which is 

 functionally but not structurally comparable to the ureter of higher 

 vertebrates (section 238), extends along the lateral border of each kidney 

 and continues to the dorsal surface of the cloaca. The urine may be 

 discharged directly through the cloaca, or it may cross and enter the 

 urinary bladder attached to the ventral surface of the cloaca. Urine may 

 be stored temporarily here and (especially in the terrestrial toads) some 

 water may be reabsorbed. 



The adrenal glands are endocrine glands that appear as a pair of 

 irregular, light-colored bands, one on the ventral surface of each kidney. 

 They produce a variety of hormones which will be considered in the 

 chapter on endocrine glands. 



