A REPRESENTATIVE VERTEBRATE. FROG 



335 



of these alveoli, where oxygen diffuses into 

 the blood and carbon dioxide is released into 

 the lung. 



The larynx (voice box) is strengthened by 

 cartilages. Across it are stretched two elastic 

 bands, the vocal cords (Fig. 218). The 

 croaking of the frog is produced by the vibra- 

 tions of the free edges of the vocal cords, 

 due to expulsion of air from the lungs. The 

 laryngeal muscles regulate the tension of the 

 cords, and hence the pitch of the sound. 

 Many male frogs have a pair of vocal sacs 

 which open into the mouth cavity. (Fig. 

 217); they serve as resonators to increase the 

 volume of sound. 



The subject of respiration in vertebrates is 

 treated more fully in Chapter 32. 



Circulatory system and 

 internal transport 



The chief function of the circulatory sys- 

 tem is to distribute body fluids to all the 

 cells, maintaining the tissue fluid which 

 bathes them in about the same state at all 

 times. 



The circulatory system of the frog consists 

 of a heart, arteries, veins, and lymph spaces. 

 The liquid portion of the blood, which is 

 known as plasma, contains three kinds of 

 corpuscles: red corpuscles (erythrocytes), 

 white corpuscles (leucocytes), and spindle 

 cells (thrombocytes). The blood plasma 

 carries food and waste matter in solution. 

 It coagulates under certain conditions, form- 

 ing a clot of fibrin and corpuscles, and a 

 liquid called serum. The power of coagula- 

 tion is an adaptation of great importance. 



since the clot soon closes a wound and thus 

 prevents bleeding to death. 



The red corpuscles (erythrocytes) are el- 

 liptical, flattened cells (Fig. 219) containing 

 a respiratory pigment called hemoglobin. 

 Hemoglobin combines with oxygen in the 

 capillaries of the respiratory organs and gives 

 it out to the tissues of the body. The white 

 corpuscles (leucocytes) are of several types 

 (Fig. 219); they vary in size, and most are 

 capable of independent amoeboid move- 

 ment. Certain kinds (phagocytes) are of 

 great value to the animal since they engulf 

 small bodies such as bacteria, frequently pre- 

 venting the multiplication of pathogenic or- 

 ganisms and helping to overcome infectious 

 diseases. White corpuscles also aid in the re- 

 moval of broken-down tissue. The throm- 

 bocytes are usually spindle-shaped. They are 

 unstable; and when brought in contact with 

 foreign substances, they break down, releas- 

 ing an enzyme, thrombin, which changes 

 fibrinogen into the insoluble fibrin so neces- 

 sary for blood clotting. Blood corpuscles 

 arise principally in the marrow of the bones. 

 They also increase in numbers by division 

 while in the blood vessels. Some white cor- 

 puscles are probably formed in the spleen, a 

 gland in which worn-out red corpuscles are 

 destroyed. 



The heart (Fig. 220) is the central pump- 

 ing station of the circulatory system. It is 

 composed of a conical, muscular ventricle; 

 two thin-walled atria,* one on the right, the 



* Auricles, according to the old tenninology. In 

 human anatomy, only the ear-shaped lobe of the 

 atrium is called the auricle. The plural of atrium is 

 atria. 



Erythrocyte Lymphocyte Eosinophil Basophil Neutrophil Thrombocyte 



Figure 219. Types of blood cells of the frog. 



