334 GENERAL ZOOLOGY 



or rectum, which passes without change of diameter into 

 the terminal cloaca. Between the lobes of the liver lies a 

 large gall bladder. Another gland, the pancreas, also fur- 

 nishes a digestive fluid. The spleen is red and globular. 



The Circulatory and Respiratory Systems. The heart in- 

 closed in its sac, the pericardium (Fig. 173), has one chamber 

 more than the heart of the perch, by the division of the 

 auricle into two parts, a right and a left auricle. The blood 

 is aerated in the lungs, the walls of which are traversed by 

 the capillaries of the blood system. The lungs communicate 

 with the exterior by means of the windpipe, or trachea, 

 opening into the mouth by a narrow slit, the glottis. 



Owing to the additional chamber in the heart and the 

 presence of lungs, the course of circulation in the frog is 

 somewhat different from that in the perch. The aerated 

 blood returned from the lungs by the pulmonary veins is 

 poured into the left auricle ; the nonaerated blood from all 

 over the body is returned to the right auricle. The auricles 

 contract at the same instant, forcing both venous and arte- 

 rial blood into the ventricle, which in turn contracts before 

 there has been much mixing of the two kinds of blood, 

 emptying its contents into the arterial circulation, the be- 

 ginnings of which are shown in Fig. 173. As the arterial 

 system takes its rise from the right side of the ventricle, the 

 first blood to enter the arteries is nonaerated. Owing to 

 the less pressure in the arteries which supply the lungs and 

 skin, and the presence of valves which cut the blood off 

 from easily entering the arteries that supply the head and 

 body, most of the nonaerated blood goes to the lungs and 

 skin. The blood which follows is a mixture of aerated 

 and nonaerated blood. When the pressure is raised by 

 the presence of nonaerated blood in the capillaries of the 

 lungs and skin, this blood forces the valves aside and 

 makes its way to the different parts of the body, except the 



