VERTEBRATES. 



49 



upon and altered by the hepatic cells, before it is drained off by the branches of the 

 hepatic vein. 



We have now to consider the changes which would take place when a fish began 

 to use its air-bladder, in addition to its gills, for respiratory purposes. Amia is able 

 to live out of water for some considerable time, undoubtedly because it can change 

 the air in its air-bladder, and thus aerate the large quantity of blood distributed to its 

 walls. Although the blood so aerated is mixed with the unaerated blood returning to 

 the heart from tlie rest of the body, it is obvious that a sufficient change is taking place 

 as long as tlie blood flows freely through the third and fourth gill-arches. Such a free 

 circulation in the gills is dependent on their being kept moist. While the fish is in 

 water it is unlikely that the air is changed in the air-bladder so frequently, but it is 

 obvious, nevertheless, that the blood returned from the air-bladder must be richer in 

 oxygen than that which has only been through the gills. 



Now in the Dipnoans, which possess a very similar disposition of the vessels, an 

 arrangement exists whereby the purer blood from the air-bladder is kept separate from 

 the blood returning from the rest of the body, and is sent through the more anterior 

 gills, so that the head receives better 

 aerated blood than does the rest of 

 the body. It is to be understood that 

 neither in Dipnoans nor in Amia are 

 the two methods of respiration equally 

 active simultaneously, but rather that 

 they alternate in accordance with the 

 requirements of the surroundings. 



The arrangement referred to above 

 consists in a subdivision of the atrium 

 into a right and left auricle, and of the 

 conus arteriosus into a right and left 

 passage, the latter carrying the purer 

 blood from the left auricle to the ante- 

 rior gills. This arrangement, which is 

 not entirely absent in Amia, is the first 

 step towards a complete separation of 

 the circulation in the lungs from that 

 in the rest of the body, as in birds and 

 mammals. Every conceivable condi- 

 tion intermediate between that in the 

 Dipnoans and that in the mammals is 

 to be found either in the embryos or 

 the adults of the amphibians and rep- 

 tiles. If we compare the heart and great vessels of a lizard with those of Amia, 

 we shall at once be able to trace the homologous parts. 



With the change from an aquatic to a terrestrial life, the gill-filaments disappear, 

 and also the respiratory net-work in these, except where a functionless net-work, or 

 rete mirabile, indicates their former position. Generally, the afferent and efferent 

 branchial arteries of each gill are represented by a continuous aortic arch, uninter- 

 rupted by any net-work. Five pairs of these aortic arches are present in the embryos 

 of all higher forms, the second of which corresponds to the trunk vessels of the first 



VOL. III. — 4 



Fig. 44. — Heart of lizard from beneath; tlio aerated blood 

 indicated by dots, tlie unaerated by crosses; a, abdominal or 

 descending aorta; c, carotid artery; I, left auricle; L, 

 lungs ; pa, pulmonary artery; ^y, pulmonary vein; r, right 

 auricle; s, subclavian artery; f, trachea; v, ventricle; //, 

 ii/, ly, remnants of branchial arches. 



