316 THE RISE OF ANIMAL LIFE 



tricle contracts, the "used" blood passes 13-37). The blood from the hmd legs has 

 out first. To further aid the separation of an alternate course in getting back to the 

 the blood there is a longitudinal valve lo- heart. It may pass via the kidneys through 

 cated in the conns arteriosus which tends the renal portal system, or via the liver 

 to direct the first blood into the pulmonary through the hepatic portal system. A portal 

 arteries and the later blood into the sys- system is a system of veins which starts and 

 temic arches and head regions where it ends in capillaries. The frog, like other 

 should go because it is richer in oxygen, lower vertebrates, has two such systems. 

 There is some mixing of the blood poor in whereas man and the higher vertebrates 

 oxygen with that rich in oxygen in this sys- have retained only the hepatic portal sys- 

 tem, and it is not as efficient as that found tem. It can be seen that this system is most 

 among the higher forms where separation is important in carrying the blood heavily 

 complete. laden with food to the liver where it can be 

 Arteries. There are three pairs of large stored and otherwise processed. If it were 

 arteries leaving the heart: the pulmocuta- not for this short circuit much of the gen- 

 neous which goes to the lungs and skin, eral circulation would be bogged down 

 the systemic arches which join and become with sugar and other food products. The 

 the dorsal aorta, and the carotids which go two precavas and the single post cava veins 

 to the head and neck regions (Fig. 13-36). enter the sac-like sinus venosus through 

 Each of these vessels divides many times three openings before proceeding on to the 

 until a network of capillaries is formed, right auricle. Blood coming from the lungs 

 and these networks supply all portions of in the pulmonary veins empties into the left 

 the body with oxygen and food. auricle, thence into the ventricle where it 



Capillaries. The arteries terminate when joins the blood from the sinus venosus. 

 their walls become one cell layer in thick- Blood. The plasma of the frog's blood is 



ness. Through vessels of this diameter very similar to higher as well as lower 

 blood cells can only pass single file, and forms, but the cells that float in it are some- 

 these tiny, thin-walled tubes are the capil- what different. The red cells are large, oval, 

 laries. There are a great many capillaries nucleated cells, but at some seasons of the 

 in all of the tissues of the frog body just as year many of the cells are without nuclei as 

 there are in the tissues of man, and it is dif- are the red blood cells of mammals. There 

 ficult to injure a portion of the skin any- are several types of white cells which vary 

 where without breaking one of these tiny somewhat from those found in human 

 vessels. These are the most important tubes blood. The blood also contains small spin- 

 of the entire vascular system because it is die cells which are concerned with blood 

 through the capillary walls that food and clotting. 



oxygen can get to the cells. The passing of Breathing system. The tadpole breathes 

 the corpuscles through the capillaries 'can by means of gills much the same as fish do. 

 be easily observed under the microscope. As it metamorphoses into the adult frog it 

 The larger vessels in such a preparation are gradually loses its gills and develops a pair 

 the arterioles and venules, which can be of lungs. The larynx, formed from the car- 

 distinguished from each other by the fact tilages that were used earlier to support the 

 that the blood flows in spurts in the arteri- gill arches, is located at the point of junc- 

 oles and only gently in the venules. tion with the mouth cavity, and contains 



Veins. After leaving the capillaries, the the vocal cords which, when vibrated, pro- 

 vessels become veins which carry the blood duce the characteristic sounds of the frog, 

 back to the heart. These grow larger and sounds which vary with the different spe- 

 f ewer as they approach the heart ( Fig. cies and are used as means of identification. 



