AIR-BLADDER OF FISHES. 



279 



by the pulmonary veins (ib. p, />'). A mixed venous and arterial 

 blood is thence distributed to the system, and again to the air-bladders. 

 True arterial blood exists only in the pulmonary veins, and unmixed 

 venous blood only in the system of the venaj cava? ; whence the ne- 

 cessity, apparently, for that peculiar arrangement by which the ar- 

 terial blood is conveyed directly to the ventricle by the pulmonary vein. 

 When the Lepidosiren resumes its true position as a fish, the bran- 

 chial circulation is vigorously resumed, a larger proportion of arterial- 

 ised blood enters the aorta, and both the nervous and muscular 

 systems receive the additional stimulus and support requisite for the 

 maintenance of their energetic actioTis. 



Anatomists and physiologists are not yet unanimous as to the ho- 

 mologies and analogies of the respiratory organs of fishes. Indeed 

 the essential distinction of those relations has seldom been clearly 

 kept in view. When we read in the latest edition of the Comparative 

 Anatomy of Cuvier : " the gills are the lungs of animals absolutely 

 aquatic*;" and, with regard to the cartilaginous or osseous supports 

 of the gills, " they are in our opinion, to the gills of fishes, what the 

 cartilaginous or osseous tracheal rings are to the lungs of the three 

 superior classes "j" : " we are left in doubt whether it is meant that the 

 gills and their mechanical supports merely perform the same function 

 in fishes which the lungs and windpipe do in mammals, or Avhether 

 they are not also actually the same parts differently modified in re- 

 lation to the different respiratory media of the two classes. Geoffrey 

 St. Hilaire leaves no doubt as to his meaning where he argues that 

 the branchial arches of fishes are the modified tracheal rings of the 

 air-breathing classes ; we perceive that he is enunciating a rela- 

 tion of homology. The truth of his proposition will be best tested 

 by first considering the homologies of the air-bladder of fishes. 

 Dr. Peters, 'Prof. Hyrtl, and others, who have prosecuted the 

 anatomy of the Lepidosiren since Dr. Bischoff" advocated its rep- 

 tilian nature, have confii'med my previous determination of that 

 genus to the class of fishes : and it may be presumed that its gela- 

 tinous chorda dorsalis, its vertebral inferior transverse processes 

 (parapophyses), the normal attachment of the scapulae to the occiput, 

 the branchiostegal covering of the permanent gills, the opercular 

 bones, the absence of pancreas, the presence of a spiral intestinal 

 valve, the relative position of the anus, the extra-oral nasal sacs, the 



* " Les branchies sont les poumons des animaux absolument aquatiques." (xiii. 

 t. vii. p. 16 1.) 



f " Elles sont, a notre avis, aux branchies des poissons, ce que les cerceaiix carti- 

 lagineux oii osscux des voics aeriennes sont aux poumons des trois classes sui)e- 

 rieures." (Ib. p. 177.) 



T 4 



