CHAPTER IX 



THE FROG (continued) : RESPIRATION AND EXCRETION 



IN the fifth chapter it was pointed out that a continual 

 waste of substance goes on in the frog's body, the lost 

 material taking the form of three chief waste-products or 

 products of excretion, water, carbon dioxide, and urea. 

 It was further stated that these substances are got rid 

 of by means of the lungs, the kidneys, and the skin. 



The Organs of Respiration. At their anterior ends the 

 two lungs open into a small, thin-walled chamber (Fig. 44, 

 I. tr. c), which, as it corresponds both with the larynx 

 or organ of voice, and the trachea or windpipe in our- 

 selves, is called the laryngo-tracheal chamber : it com- 

 municates with the pharynx through the glottis (gl). 

 The walls of the chamber and the edges of the glottis are 

 supported by cartilages (ar) . 



The structure of the lung is best made out by distend- 

 ing it with air, and then placing it in formaline or spirit 

 until thoroughly hardened : its walls contain so much 

 elastic tissue that if cut when fresh it contracts 

 immensely, and its structure is then difficult to see. 

 The inner surface of the lung is raised up into a con- 

 plex network of ridges (A, r. Ing), which project into 

 the interior and produce the appearance of an irregular 

 honeycomb. All these ridges are abundantly supplied 



141 



