24 MORPHOLOGY OF THE VEETEBBATA. 



ALIMENTARY TRACT. 



The tongue is hard, and, like the mandibles, is ensheathed in horn. 

 It essentially consists of a prolongation of the glossohyal bone, and 

 is chiefly adapted to fulfil the function of a prehensile organ, in 

 association with the beak. Salivary glands are developed on each 

 side of the interior of the mouth, and pour their secretion on 

 the food prior to deglutition. It is destitute of papillse, except at 

 its base : these papillae are supplied by filaments of the glosso- 

 pharnyngeal nerve. No branch of the fifth nerve goes to the 

 tongue. 



The oesophagus is long, and as it passes down it inclines 

 towards the right side, being partially covered by the trachea, 

 and connected to the surrounding parts by a loose cellular tissue. 

 It is dilated into a pouch : the crop, which is double, consisting of 

 two lateral oval cavities, lodged between the coalesced clavicles. 

 The change the food undergoes in the crop is well known : if a 

 pigeon be allowed to swallow a great number of peas, they will 

 swell to such an amount as almost to suffocate it. Hunter made 

 many observations on the crop of pigeons, which takes on a 

 secreting function during the breeding season, for the purpose of 

 supplying young pigeons in the unfledged state with a suitable 

 diet for their tender condition. An abundant secretion of a milky 

 fluid is poured out into the crop, and mixed with the macerating 

 grains. This is a near approach to the mammary secretion of the 

 mammalia. 



The oesophagus, a short distance below the crop, opens into the 

 proventriculus, which corresponds to the pyloric of the stomach 

 of higher animals — a gastric juice being secreted from glands in 

 its walls. At its lower end the proventriculus embouches into 

 the gizzard, a thick-walled muscular sac, situated below the liver 

 on the left side of the abdomen. Its upper end has two apertures: 

 one of these is of large size, communicating with the proven- 

 triculus ; the second is close to and on the right side of the pre- 

 ceding, leading to the duodenum. Below these apertures, the 

 cavity of the gizzard extends to form a cut de sac. At the middle 

 of the anterior and posterior parts of the cul de sac there is a 

 tendinous centre, from which the muscular fibres radiate. 



In opening the gizzard, notice that the epithelium is developed 

 into a hard horny coat, adapted for triturating the food. 



The pyloric orifice of gizzard is guarded by a valve. The 

 intestines reach from the stomach to the cloaca : in relative length 

 they are short. The canal is divided into small and large 

 intestines, by the insertion of two short coeca, which are opposite 

 one another. In the crown pigeon these coeca are altogether 

 wanting. 



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