484 



COLLEGE ZOOLOGY 



pocal cords due to the 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. 411, 5); they serve as 

 resonators to increase the volume of sound. 



The Circulatory System. — The circulatory system of the 

 frog consists of a heart, arteries, veins, and lymph spaces. The 

 blood is a plasma containing three kinds of corpuscles, — red 

 corpuscles, white corpuscles, and spindle cells. The blood 

 plasma carries food and waste matter in solution. It coagu- 

 lates under certain conditions, forming a clot of fibrin and cor- 

 puscles, and a liquid called serum. The power of coagulation 



is of decided benefit, 

 since the clot soon 

 closes a wound and 

 thus prevents loss of 

 blood. 



The red corpuscles 

 (erythrocytes, Fig. 

 413 a, a) are elliptical, 

 flattened cells con- 

 taining a substance called hcemoglobin. Haemoglobin combines 

 with oxygen in the capillaries of the respiratory organs and 

 gives it out to the tissues of the body. The white corpuscles 

 (leucocytes, Fig. 413 a, b) are ameboid in shape, vary in size, 

 and are capable of independent movement. They are of 

 great value to the animal, since they engulf small bodies, such 

 as bacteria, thereby frequently preventing the multiplication 

 of pathogenic organisms and consequently helping to overcome 

 germ diseases. White corpuscles also aid in the removal of 

 broken-down tissue. The spindle cells (Fig. 413 a, c) are usually 

 spindle-shaped. In the springtime they develop into red cor- 

 puscles. Blood corpuscles arise principally in the marrow of 

 the bones. They also increase in numbers by division while 

 in the blood-vessels. Some white corpuscles are probably formed 



Fig. 413 a. — Blood corpuscles of the frog. 

 a, red; b, white; c, spindle cells. (From Holmes, 

 after Dekhuyzen.) 



