184 Descriptive Zoology. 



raised, air is forced into the lungs. The gullet is kept 

 firmly closed so that air does not enter it, and the glottis, a 

 longitudinal slit in the floor of the mouth just back of the 

 tongue, being opened at the right time, air freely enters 

 the lungs. The windpipe, or trachea, is extremely short, 

 dividing almost immediately to enter the two lungs. The 

 less frequent movements of the sides of the body seem to 

 be for driving air out. This is accomplished by the action 

 of the muscles of the sides of the abdomen. From the 

 above it may be understood why a frog may be suffocated 

 by having its mouth held open for any considerable time. 



The frog also breathes to a limited extent by means of 

 the skin. It has been noticed that the skin is always 

 moist, a condition necessary for this function. In cool 

 weather it will be noticed that frogs kept in aquariums 

 frequently sink to the bottom and remain there for a long 

 time. Of course the breathing movements then cease. 

 The diminished activity is accompanied by a reduced re- 

 spiratory need, and the blood,, circulating in the skin, 

 absorbs all the oxygen that is required. 



Circulation of Blood in the Frog. The heart consists of 

 one ventricle and two auricles. The right auricle receives 

 blood through the .caval veins from all parts of the body 

 except the lungs; this blood is dark because it has been 

 deprived of its oxygen by the working tissues of the body ; 

 it is loaded with impurities which the tissues have given to 

 it. The left auricle receives blood from the lungs ; this 

 blood is bright-colored from the oxygen just obtained in 

 the lungs ; it has also lost some carbon dioxid. 



The two auricles contract at the same time and .send 

 these two kinds of blood into one ventricle. How is one 

 ventricle to send the purer blood to the organs that need it, 

 and the impure blood to the organs whose work it is to 



