COMMON PIGEON. 365 



PLATE II. 



COMMON PIGEON (Columba livia), 



Dissected so as to show the main points characteristic of Aves, and the arrangement 

 of the principal muscles of flight. 



THE characters distinctive of Birds shown in this figure are the follow- 

 ing : feathers ; epidermic scales to the feet ; musculature of the wing ; 

 characters of the brain ; oesophageal crop (absent in some Birds) ; large 

 size of duodenal loop ; pancreas and the number of ducts to this gland ; 

 the two coeca ; extremely short large intestine as compared with the length 

 of the small intestine ; heart resting in a deep notch of the liver ; single 

 aorta crossing the right bronchus ; mode of division of the innominate 

 artery ; deep indentation of the lungs by the ribs ; tubular structure of the 

 lungs ; and trilobed kidney adapted to the pelvic fossae. 



There is no diaphragm as in all lower Vertebrata : the cloaca, as in 

 Crocodilia, Chelonia and Amphibia, receives separately the rectum, urinary 

 and generative ducts : the testes are permanently retained within the abdo- 

 men as in a few Mammals and all lower Vertebrata : the bladder is 

 absent as in all Lacertilia and Ophidia. 



In some Mammals a portion of the stomach is purely receptive, but no 

 Mammal developes an oesophageal crop ; and it is as rare for a Mammal 

 to possess two coeca as it is for a Bird to possess one. The only Mammals 

 the Prototheria which have a cloaca, have also a sinus urogenitalis. 



a. Right cerebral hemisphere. Its surface is smooth, contrasting with 



that of the transversely laminated cerebellum seen behind in the 

 median line. 



b. The crop, which is bilocular in the Columbidae. A window has been 



made in its right wall to show its division into two compartments. 



c. Right lobe of liver. 



d. Heart. The ventricular portion is more acutely conical in most Birds 



than in Mammals, and the auricles are smaller in relation to the 

 ventricles. 



e. Loop of duodenum in which are contained the longitudinally arranged 



lobes of the pancreas. Into this loop of intestine three ducts open 

 from the pancreas and two from the liver, which has no gall-bladder 

 in this species. Two of the pancreatic ducts open near the middle of 

 the distal segment of the duodenum close to each other and to one of 

 the gall-ducts ; the third pancreatic duct opens near the distal end of 

 the loop, and the second gall-duct near its proximal end. 

 e\. Terminal segment of small intestine ending in the large intestine at 



