THE FROG. 



95 



The two branches into which the aorta separates 

 come together beliind the iieart, forming one dorsal 

 aorta. 



The corjiuscles of the blood are 

 large and elliptical in outline, and 

 each is ijrovided with a nucleus. 

 With a compound microscope the 

 movement of blood corpuscles can 

 be seen in the capillaries of the 

 web of a frog's foot. It can also 

 be observed in the tail of a tad- 

 pole, in the tail fin of a minnow, 

 and in tlie mesentery of a living 

 frog or toad. Even with a simple 

 lens some movement ixiaj' be ob- 

 served. 



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Br,OOD cr)RPUSCT^ES OK Fiii 



The intestine opens into a short, wide sac called 

 the cloaca, which receives also the oviducts and the 

 tubes from the kidney's. The long bones of the legs 

 terminate in cartilaginous caps which are easily sep- 

 arated from the rest of the bones. 



The frog is without any riljs or diaphragm. In 

 man, these are the organs by means of which the air 

 is taken into the lungs. The frog must l)reathe in 

 another way. Under the throat or jaw is a large, 

 flat muscle called the mylohyoid muscle. When 

 this muscle is depressed, the cavity of the mouth 

 is enlarged, and air rushes in through the nos- 

 trils. The nostrils are then closed, either by valves 

 in the openings, or by the tip of the tongue, or 

 l)y both, and the mylohyoid muscle is raised. 

 The air is thereby forced down into tlie lungs. The 

 process is repeated, air coming from the lungs and 

 mingling in the mouth witli fresh air from the out- 

 side. If a frog's inouth wei'e pjroi^ped open, could 

 liebreatlie? 



