546 AMPHIBIA. 



heart, to consider how it is that the blood is propelled from 

 the ventricle along the proper channels. The right half of 

 the ventricle being nearer the right auricle, contains more 

 impure blood, and it is from the right side of the ventricle 

 that the truncus arteriosus arises. Therefore, when the 

 ventricle contracts, the blood which first fills the truncus is 

 venous. It passes along the left side of a median longitu- 

 dinal valve into the pulmonary arteries — along the path of 

 least resistance. As the pulmonary arteries become dis- 

 tended, the next quantum of blood — that which 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 openings 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 maintain- 

 ing the health of the blood, is represented by two little bddies 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 oesophageal region 

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

 a short laryngotracheal 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 



