16 SATURDAY LECTURES. 



place you will see an unpaired membranaceous air bladder, 

 which is connected by a simple, narrow tube with the oesoph- 

 agus, and in the swordtish, the tube, even, is entirely wanting, 

 and the air bladder is shut off from communication with the 

 mouth, direct or indirect. Nevertheless, did time permit, 

 1 could easily convince you that the lungs and air bladder 

 graduate into each other, and that the two truly represent, 

 or, in the language of the anatomist, are homologus with 

 each other. On the one hand, our common gar has an air 

 bladder so cellular as to be somewhat lung-like ; polypterus 

 has a still more lung-like bladder, and its relation to the 

 intestinal canal, also approximates that of a lung ; next a 

 remarkable fish of Australia, named ceratodus, has what 

 may more properly be called lungs than air bladder, and 

 related forms of South America and Africa, known as Lepi- 

 dosirenids, have as true lungs as amphibians. On the other 

 hand, the amphibians, reptiles, and mammals show a gra- 

 dation from the simple to the complex form manifest in the 

 last. 



Thus it will be apparent that the respiration of fishes and 

 whales are effected by entirely different organs, and that 

 the same organs may be modified and adapted for very dif- 

 ferent purposes. Nature is economical of her material, but 

 most ingenious and versatile in the use of it, and employs 

 the same stuff in many ways. 



In connection with the respirator}^ apparatus we very 

 naturally consider the heart, which receives the blood which 

 has coursed through the bodv, and sends it to be purified 

 and aerated b}^ the respiratory process. 



In the whale, the heart is partitioned into four cavities or 

 chambers — a right auricle and a ventricle, and a left auri- 

 cle and a ventricle — as in man. The right auricle receives 

 the blood which has circulated through the veins, and the 

 right ventricle transmits it to the lungs where it is oxygen- 

 ated and thence goes bright and purified to the left auricle, 

 and by the left ventricle is transmitted to again course 

 through the vessels of the body. In the fish, the heart 

 has onlv two chambers; an auricle collects the blood that 



