THE ALIMENTARY CANAL IN SAdROPSIDA. 263 



posterior vertical, and an external horizontal, are connected 

 with the membranous vestibule. In Aves, the anterior vertical 

 canal is very large in proportion to the others, and the adja- 

 cent crura of the two vertical canals overlap before they unite 

 with one another. 



Labial and buccal glands are developed in some Scmropsida, 

 and one of them, on each side, attains a large development in 

 the poison-glands of the venomous snakes. Well-developed 

 sublingual, submaxillary, and parotid glands appear in Birds, 

 and the sublingual glands attain an immense size in the Wood- 

 pecker. The tongue varies greatly, being sometimes obsolete, 

 as in the Crocodile and some birds (e. g., the Pelicans), some- 

 times horny and even spinose, sometimes fleshy. In the 

 snakes, and some lizards, the tongue is forked, and capable 

 of retraction into a basal sheath. In the Chamseleons, it is 

 clubbed at its extremity, and can be retracted or protruded by 

 the invaofination or inversion of its hollow stem. 



The ahmentary canal of the Sauropsida is generally divided 

 into an oesophagus, a simple stomach, a small intestine and 

 large intestine, which last always terminates in a cloaca. It 

 is invested by a peritoneal coat, which generally follows all 

 the curvatures of the intestine. But in the Ophidia^ the folds 

 of the small intestine are united by fibrous tissue, and enclosed 

 by a common sheath of peritonaeum. 



The stomach is usually a simple dilatation of the alimen- 

 tary canal, the cardiac and pyloric apertures of which are 

 remote from one another ; but, in the Crocodiles, and in most 

 Birds, the pyloric and cardiac apertures are approximated. In 

 many Crocodilia and Aves, there is a pyloric dilatation before 

 the commencement of the duodenum. 



In the Crocodilia, and in Aves, the walls of the stomach 

 are very muscular, and the muscular fibres of each side radiate 

 from a central tendon or aponeurosis. The thickening of the 

 muscular tunic of the stomach attains its maximum in the 

 graminivorous birds ; and it is accompanied by the develop- 

 ment of the epithelium into a dense and hard coat, adapted 

 for crushing the food of these animals. Birds commonly aid 

 the triturating power of this gastric mill by swallowing stones ; 

 but this habit is not confined to them, crocodiles having been 

 observed to do the same thing. 



Birds are further remarkable for the development of a 

 broad zone of glands in the lower part of the oesophagus, 

 which is usually dilated, and forms o, p)roventricidus, connected 



