498 ANATOMY OF VERTEBRATES. 



by which it becomes also analogous in function to the lungs of 

 the air-breathing Amphibia. 



The Lepidosiren annectens l inhabits a part of the river Gambia, 

 which in the rainy season overflow! extensive tracts, that are 

 again left dry in the dry season. Those which do not follow the 

 retreating waters escape from the scorching rays of the African 

 sun by burrowing in the mud, which is soon baked hard above 

 them ; but they maintain a communication with the air by a 

 small aperture, and, coiling themselves up in their cool chamber, 

 clothe themselves by a layer of thick mucous secretion, and 

 await, in a torpid state, the return of the rains and the over- 

 flowing of the mud-banks. The advent of their proper element 

 wakes them into activity : they then emerge from the softened 

 mud, swim briskly about, feed voraciously, and propagate. 



The peculiar modifications of the gills and air-bladder of the 

 Lepidosiren are precisely those which adapt them to the peculiar 

 conditions of their existence. In the inactive state into which 

 they are thrown by their false position as terrestrial animals, the 

 circulation, which would have been liable to be stopped had all 

 the branchial arteries developed gills, as in normal fishes, is 

 carried on through the two persistent primitive vascular channels, 

 fig. 312, 2 and 3. Whatever amount of respiration was requisite 

 to maintain life during the dry months is effected in the pulmonary 

 air-bladders ; its short and wide duct or trachea, the resophageal 

 orifice of which is kept open by a laryngeal cartilage, fig. 316, jf, 

 introduces the air directly into the bladders : the blood transmitted 

 through the branchial arches to the pulmonary arteries, fig. 312, /', 

 is distributed by their ramifications over the cellular surface of 

 the air-bladders,' and is returned arterialised by the pulmonary 

 veins, ib. p, 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 venae cava3 ; whence the 

 necessity, apparently, for that peculiar arrangement by which the 

 arterial blood is conveyed directly to the ventricle by the pul- 

 monary vein. When the Lepidosiren resumes its true position as 

 a fish, the branchial circulation is vigorously resumed, a larger 

 proportion of arterialised blood enters the aorta, and both the 

 nervous and muscular systems receive the additional stimulus and 

 support requisite for the maintenance of their energetic actions. 



Anatomists and physiologists have expressed different views as 

 to the hornologies and analogies of the respiratory organs of 



1 XXXIII. 



