XXIV] ALIMENTARY CANAL 625 



which pull the enlarged third visceral arch forwards. Behind the 

 tongue open the ducts of the submaxillary glands; at the 

 corners of the gape the parotid glands pour their secretion into 

 the mouth, whilst at the sides of the tongue the sublingual 

 glands open. All these glands are pouch-like outgrowths of the 

 ectoderm of the stomodaeum and secrete a mucus which assists in 

 swallowing the food, and occasionally (as in "Woodpeckers) in 

 causing the prey to adhere to the tongue. The names indicate the 

 position of the glands, as for instance, parotid (Gr. Trapa, beside, 

 ov9, WTO?, the ear). Following on the buccal cavity and indis- 

 tinguishably fused with it is the endodermal pharynx into which 

 the glottis opens, and also the persistent remains of the first pair 

 of gill-sacs, the Eustachian tubes. The pharynx leads into a long 

 gullet lying dorsal to the trachea, which eventually passes into the 

 stomach. The gullet in the Pigeon and many other birds develops 

 a large thin- walled outgrowth on the ventral side called the crop. 

 This is used as a storehouse for the food, and in the Pigeon 

 may be found full of unaltered seeds. The stomach has a most 

 characteristic form in Birds ; it is sharply divided into two regions, 

 an anterior egg-shaped one called the proventriculus, and a large 

 posterior flattened one called the gizzard. In the walls of the 

 proventriculus are found the pepsin-forming glands, while on the 

 other hand the endoderm of the gizzard develops a horny lining 

 which is thin in Birds that live on an animal diet, but very thick in a 

 grain-eating Bird like the Pigeon, where it forms upper and lower 

 hardened plates. When by the contraction of the greatly thickened 

 visceral muscles of this part of the alimentary canal the upper and 

 lower plates are brought together, a crushing-mill is produced by 

 which the food is broken up. The action of this mill is assisted by 

 the habit which many Birds possess of swallowing fragments of 

 stone. A collection of these, sometimes including fragments of 

 glass, may be found on opening the gizzard of a Pigeon. It is a 

 great development of this habit which has earned for the Ostrich its 

 reputation of flourishing on a diet of nails, penknives and match- 

 boxes. The liver in Birds is remarkable for possessing two ducts, 

 one opening as usual close to the pyloric end of the stomach and 

 one into the distal end of the first loop of the intestine. The 

 pancreas of Birds has from one to three ducts. It has been recently 

 proved that the pancreas of all Amniota originates as three pouches 

 of the intestine behind the pouches which give rise to the liver a 

 dorsal pouch and two ventral pouches. All these pouches divide 

 S. & M. 40 



