62 MORPHOLOGY OF THE VERTEBIIATE BODY 



The Respiratory System. — By respiratory organs one com- 

 monly means such parts of an animal as the lungs or gills, where 

 oxygen enters the blood, although the essential fact of respira- 

 tion is the utilization in all parts of the body of the oxygen car- 

 ried from the respiratory organs by the blood. A frog can live 

 for some months after its lungs have been removed, if it is not kept 

 at too high a temperature. The skin of this animal is, in fact, 

 a part of its respiratory system, although we shall consider only 

 the lungs in this connection. The lungs of the frog consist of two 

 simple sacs connected with the mouth cavity by means of the 

 larynx and the glottis. Unlike the lungs of man and other warm- 

 blooded animals, they are not subdivided into many complex 

 outpocketings, the alveoli, but exhibit simpler foldings of their 

 inner surfaces. 



The mechanism which causes the air to pass in and out of the 

 lungs differs in man and the frog, because of the absence of a dia- 

 phragm in the latter animal. In man, the mechanics of the process 

 would resemble the sucking of air in and out of a pipette, if the 

 single rubber bulb of the pipette were lined with another more 

 delicate bulb, fused with the outer bulb at its neck and lying close 

 against it in every expansion and contraction. In such a comparison 

 the outer bulb would then represent the body wall of the chest ; the 

 inner bulb, the lungs ; and the glass tube of the pipette, the trachea 

 leading to the mouth. Expanding the cavity of the outer bulb 

 draws air into the cavity of the inner bulb; contracting the outer 

 bulb forces this air outward. There is never any space between 

 the two bulbs. In like manner an expansion of the chest cavity, 

 whether by elevation of the ribs or by a downward movement of 

 the diaphragm, draws air into the lungs, while contraction of the 

 chest forces the air outward. 



In the frog, the mechanism is wholly different. If one watches 

 the respiratory movements of the living animal, it will be observed 

 that the nostrils open and close in relation to a rhythmic beating 

 of the floor of the mouth, and that each beat of the mouth corre- 

 sponds to a slight movement of the walls of the body. While not 

 observable externally, it can also be shown that the glottis opens 

 and closes in relation to these other movements. When observed 

 for a brief period, the walls of the body are seen to swell by almost 

 imperceptible stages and then suddenly to collapse, at which time a 

 considerable amount of air is discharged from the opened nostrils. 



