330 GENERAL ZOOLOGY 



The Circulatory, Respiratory, and Excretory Systems. The 

 heart inclosed in its sac, the pericardium (Fig. 165, 17), has 

 one chamber more than the heart of the perch, by the divi- 

 sion of the auricle into two parts, a right and left auricle 

 (Fig. 165, 20, 18). The blood is aerated in the lungs (Fig. 165, 

 25, 26), 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 (Fig. 165, 24), opening 

 into the mouth by a narrow slit, the glottis (Fig. 165, 23). 



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 non-aerated blood from all 

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

 contract at the same instant, forcing both venous and arterial 

 blood into the ventricle (Fig. 165, 19), which in turn con- 

 tracts before there has been much mixing of the two kinds 

 of blood, emptying its contents into the arterial circulation, 

 the beginnings of which are shown in Fig. 165, 21 and 22. 

 As the arterial system takes its rise from the right side 

 of the ventricle, the first blood to enter the arteries is non- 

 aerated. 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 non-aerated blood goes to the 

 lungs and skin. The blood which follows is a mixture of 

 aerated and non-aerated blood, and the pressure being raised 

 by the presence of non-aerated' 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 head. 

 In the course of the artery leading to the head is a structure 

 (the carotid gland) which temporarily obstructs the flow of 

 blood till the body-arteries have become filled ; and the supply 



