The Frog 63 



internal or posterior nares into the mouth cavity. The valves then 

 close; the floor of the mouth is raised, and the air is forced through 

 the larynx ( ) into the lungs. 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. 



End of Bronchiole The lungs themselves (Fig. 15) are formed of 



minute chambers called alveoli, ( ) 



the walls of which are filled with little blood capil- 

 laries. The larynx is strengthened by five cartilages, 

 ( ) across which the vocal 



cords are stretched. The expulsion of air from the 

 Alveoli j un g S across th e f r ee ends of the vocal cords pro- 

 Fig, isf Alveoli of Lungs. duces the sound known as croaking. The laryngeal 

 muscles regulate the tension of the cords which causes 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, which has been digested 

 and 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 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 to the extent that human beings do, because 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 

 animal forms, which lie between the dorsal peritoneum and body-wall. 

 The kidneys are abundantly supplied with blood vessels, though they, 

 themselves, are composed of connective tissue. The fact that so many 



