434 Animal Biology 



caval) with its branches^ (2) the right anterior vena cava (right pre- 

 caval) and its branches and (3) the left anterior vena cava (left pre- 

 caval) and its branches. The blood from the sinus venosus enters the 

 right auricle. The right and left auricles send their blood into the one 

 ventricle which forces its mixture of oxygenated blood (from the left 

 auricle) and nonoxygenated blood (from the right auricle) into the 

 truncus arteriosus through three pocket-shaped semilunar valves. The 

 venous part of the frog circulatory system is shown in the diagram on 

 page 433. Compare and contrast the diagram of the venous system with 

 the diagram of the arterial system. 



Frog blood (Fig. 12) is quite complex and consists of: (1) Oval, 

 biconvex, nucleated red blood corpuscles (erythrocytes) which contain 

 hemoglobin. The latter unites temporarily with oxygen in the lungs and 

 skin to form oxyhemoglobin, which in turn gives up its oxygen to cells 

 and tissues when or where it is needed. (2) Amoeboid white blood 

 corpuscles (leucocytes) which are able to move independently and are 

 of different sizes. They pass through the walls of blood vessels and tis- 

 sues. They destroy bacteria and other organisms by ingesting them, 

 thus serving to prevent infections. (3) The spindle cells are frequently 

 spindle shaped and upon their disintegration assist in the clotting of blood. 

 Blood corpuscles originate principally in the marrow of the bones but 

 may also increase in numbers by division within the blood vessels after 

 being formed. (4) The plasma or liquid part of the blood carries foods, 

 wastes, proteins, mineral salts, etc. Blood coagulates, especially after in- 

 juries, to form a clot which includes fibrin, red and white corpuscles, tis- 

 sue cells, etc. 



Respiration.— In the earlier, tadpole stages, external gills are present 

 for respiration, but these are later covered to form internal gills which 

 communicate with the exterior through a small opening. The internal 

 gills are eventually absorbed and typical lungs develop in the air-breath- 

 ing adult frog. In the adult frog respiration takes place through the skin 

 and lungs, probably more through the former than the latter. During 

 hibernation the lungs are inactive, yet skin respiration continues, even 

 though the rate may be reduced (Fig. 208). In lung respiration the air 

 is admitted into the mouth cavity (Fig. 212) from the outside through 

 the external nares (nostrils) and then through the slitlike glottis into the 

 short, tubular larynx: from the latter the air passes into the trachea 

 (windpipe) and finally into the thin-walled, saclike, paired lungs. The 

 lungs are ovoid, distensible, and internally divided by folds (septae) into 

 a number of compartments known as alveoli (al-ve'oli) (L. alveolus, 



