XII.] 



THE EXCRETORY ORGANS. 



465 



from other tubes, which again spring at right angles from the 

 bronchi or primary divisions of the windpipe. 



The lungs may be cellular throughout their extent, while 

 yet the bronchi do not give off branches within them, as in 

 Chelonia and Crocodilia. 



They may be cellular only near their root, the more post- 

 axial part of the lung being a simple membranous air-bag, as 

 in Serpents. 



Part of the arterial blood, carried by the pulmonary artery, 

 instead of going to the lung may be conveyed by small 

 branches to the oesophagus, as in Menopoma, Amphiuma, 

 and Siren. 



Part of the venous blood of the 

 lungs, instead of returning direct to 

 the heart, may enter the veins of the 

 trunk, as in Proteus. 



When there is an air-sac which does 

 not both receive blood directly from 

 and return it directly to the heart 

 /.<?., when there is no true pulmonary 

 circulation such an air-sac (whether 

 single or double) is termed a swim- 

 bladder, or air-bladder, and a struc- 

 ture of the kind is found in most 

 Teleostean and in all Ganoid Fishes. 



A swim-bladder, however, may 

 closely approximate to a lung, as in 

 Ceratodus, where, though blood is 

 not brought to that bladder directly, 

 as in Lepidosiren, yet blood is sent p IG . 

 directly to the heart by a pulmonary 

 vein which opens into the atrium of 

 the heart, or chamber representing 

 the (as yet undifferentiated) auricles. 



The swim-bladder may present a great variety of conditions. 



Thus, it may be double and connected with the ventral part 

 of the cesophageal portion of the alimentary canal by a short 

 tube, the ductus pneumaticus, as in Polypterus j or single and 

 connected more with the side of the same part, as in Cera- 

 todus and Erythrinus. It may be connected with the dorsal 

 part of the cesophagus, as in Lepidosteus and the Carp ; or 

 with the cardiac part of the stomach, as in the Herring. 



The ductus pneumaticus may be long, narrow, and bent, as 

 in the Carp ; or short, straight, and wide, as in Lepidosteus. 

 H H 



393. DIAGRAM OF A 

 LOBULE OF THE LUNG OF A 

 BIRD, GREATLY MAGNIFIED. 



(After Thomas Williams.} 



