THE FROG -63 



have already been mentioned, then close ; the floor of the mouth is raised, 

 the air being" forced through the larynx ( ) to the 



lungs themselves. The contraction of the body-wall forces the air back 

 from the lungs into the mouth. It is interesting to note that the glottis 

 closes, while the floor of the mouth alternately raises and lowers thus 

 drawing in and expelling air through the nares into the mouth cavity 

 by what are called throat movements. 



The lungs themselves (Fig. 15) are formed of 

 minute chambers called alveoli ( ) 



tne wa lls of which are filled with little blood capil- 

 laries. The larynx is strengthened by five carti- 

 lages, ( ) across which the vocal 

 cords are stretched. The expulsion of air from the 

 lungs across the free ends of the vocal cords pro- 

 Aiveoii of Lungs. duces the sound known as croa kmg. The laryngeal 



muscles regulate the tension of the cords, causing the particular pitch 

 of the sound made. 



Male frogs often have a pair of vocal sacs opening into the mouth 

 cavity, serving as resonators ( ) and increasing 



the volume of the sound. 



THE EXCRETORY SYSTEM 



The food taken into the body is said to be ingested. The part of 

 the food which is actually taken into the blood as nutriment is said to 

 be digested, and that part of the food which passes directly through .the 

 body without becoming a part of it is said to be egested. Every living 

 cell ingests and must assimilate food in order to live ; consequently, it 

 must also get rid of that material which has already served a nutrient 

 purpose, and this getting rid of a substance that has been digested and 

 that has served a purpose is called excretion. This word must not be 

 confused with secretion, which means that a substance is given off from 

 the cell or gland which is to be used again by some part of the body. 

 The waste matter that is eliminated from the body in the form of carbon 

 dioxide is thrown off through the organs of respiration, but the solid 

 products have specialized organs for their removal. The skin serves as 

 such an organ to a small extent. The frog does not use the skin in this 

 way anywhere nearly to the extent that human beings do, because the 

 amphibia do not possess sweat glands. The liver and the walls of the 

 intestine are also excretory in character. 



The most important organs for excretory purposes, however, are 

 the kidneys, two oval, flattened dark red bodies lying behind the peri- 

 toneum in the dorsal portion of the body-cavity. It is well to know that 

 the kidneys are about the only abdominal organs, even in the higher 

 forms, that lie between the dorsal peritoneum and body- wall. The kid- 

 neys are abundantly supplied with blood vessels, though they, them- 

 selves, are composed of connective tissue. The fact that so manv blood 



