4S8 AMPHIBIA. 



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 longitudinal valve into the pulmonary 

 valves — along the path of least resistance. As the pulmonary 

 arteries become distended, 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 longi- 

 tudinal 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 arte- 

 rialised 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 textbook of Practical Biology, by Huxley 

 and Martin, Howes and Scott.) 



Spleen, Thyroid, and Thymus. — The spleen, whose func- 

 tion 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, of 

 uncertain significance, is represented by two little bodies 

 near the roots of the aortic arches. The thymus, perhaps 

 originally associated with residues of branchial 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. 



The lungs arise as outgrowths of the oesophageal region of 

 the gut, and are developed some time before they come 

 into use. They are connected with the back of the 

 mouth, as we have already noticed, 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 



