546 AMPHIBIA. 



has been mixed in the middle of the ventricle is driven 

 forwards, and passes on the right side of the longitudinal 

 valve into the aortic arches. " And, as the truncus becomes 

 more and more distended, the longitudinal valve, flapping 

 over, tends more and more completely to shut off the open- 

 ings of the pulmonary arteries, and to prevent any blood 

 from flowing into them. Finally the last portion of blood 

 from the ventricle, representing the completely arterialised 

 blood of the left auricle, which is the last to arrive at the 

 opening of the truncus, passes into the carotid trunks, and 

 is distributed to the head." (The last two sentences are 

 quoted from the " Text-book of Practical Biology," by 

 Professors Huxley and Martin, Howes and Scott.) 



Spleen, Thyroid, and Thymus. The spleen, which is probably, as in 

 some other animals, concerned with blood making, is a small red organ 

 lying in the mesentery near the beginning of the large intestine. The 

 thyroid, which is believed to have something to do with maintaining the 

 health of the blood, is represented by two little bodies near the roots of 

 the aortic arches. The thymus, perhaps originally associated with the 

 gill-clefts, lies on each side just behind the angle of the lower jaw. 



Respiratory System. 



The larval frog breathes at first through its skin, then by 

 external gills, and, finally, by internal gills. The adult frog 

 breathes chiefly by its lungs, but some cutaneous respiration 

 is still retained, for even without its lungs a frog may live 

 for some time, and it does not use them when hibernating. 



The lungs arise as outgrowths of the cesophageal region 

 of the gut, and are connected with the back of the mouth by 

 a short laryngo-tracheal tube, whose slit-like aperture is the 

 glottis. Each lung is a transparent oval sac, with muscle 

 fibres in its walls. The cavity is lessened by the spongy 

 nature of the internal walls, which form numerous little 

 chambers bearing the fine branches of blood vessels. 



In respiration, the mouth is kept shut, and air passes in 

 and out through the nostrils. A frog will die of asphyxia 

 if its mouth be artificially kept open for a considerable time. 

 When the floor of the mouth is lowered, and the buccal 

 cavity thus increased, air passes in. When the elastic lungs 

 and the muscles of the sides of the body contract, air passes 

 out. When the nostrils and also the opening of the gullet 



