COMMON PIGEON. 53 



The tongue is of fair size. It has been discovered by Fraisse (Z. A. iv. 1881, 

 p. 310), that in the embryo Duck there are embryonic feathers developed on the 

 tongue which are arrested in development. The crop in the Pigeon is remark- 

 able for its large size and bilateral symmetry. Gadow has distinguished between 

 a true crop with glandular walls and a * Haut ' or ' Schlund ' crop with non-glan- 

 dular walls. The former exercises a chemical action on the food and occurs in 

 the Fowl and Pigeon and in their congeners ; whilst the latter exercises no such 

 action and is simply a storehouse for food swallowed, e. g. in many Ducks, Cassowary, 

 &c. But the researches of Hasse (Zeitschrift fur Rationelle Medizin, xxiii. 1865) 

 proved long ago that the upper part of the oesophagus and the crop itself are non- 

 glandular in the Pigeon, whereas the portion of the oesophagus below the crop, 

 like the proventriculus, is provided with glands. The crop, and the upper as well as 

 the lower part of the oesophagus, are lined by a many-layered epithelium the lower 

 cells of which are granular and plump, but as they pass to the surface become 

 flattened out yet not cornified. If the surface of the crop is scraped a small 

 amount of a whitish liquid can be collected at all times. The amount is greatly 

 increased in both sexes for about the first eight days after the hatching of the 

 young, which are fed with the so-called ' pigeon's milk ' regurgitated by the parent 

 bird into the mouth of the young. It is a milky liquid containing cheese-like 

 solid morsels. Hasse found that at this time the epithelium of the upper parts of 

 the oesophagus and of the crop, but especially its side parts was much thickened, 

 the bloodvessels dilated and full of blood. He also found that the cells of the 

 epithelium undergo rapid division : are granular, and contain abundant fatty gran- 

 ules : that they are set free in masses which break down partially. The rem- 

 nants form the cheese-like morsels, whilst the fatty cells set free give the 

 liquid a milky look. The cells in the masses retain their nuclei, those set free 

 have either lost them or show them undergoing fatty degeneration. The ' milk ' 

 collects within the crop whence it is expelled by the action of two muscles which 

 spring from the upper part of the clavicles and are inserted into the skin ven- 

 trally. The physiological properties of the fluid do not appear to have been fully 

 investigated. It is doubtful, perhaps, whether the small amount of it present at times 

 other than the breeding season has any chemical effect on the food. The swelling 

 of grain, peas, &c., in the crop may be due only to the action of moisture and 

 warmth, and is therefore a physical effect. It is stated by Hasse that a similar 

 milky secretion occurs in some species of Parrots. 



The lower oesophagus has an epithelium similar to that of the upper find of 

 the crop, but there are a small number of glands in the mucous membrane with 

 an alkaline or neutral secretion. In the proventriculus the epithelium is reduced 

 to a single layer of columnar cells. The glands of this region secrete the acid 

 gastric juice. In the Pigeon they are small and simple : in the Fowl and Goose 

 they have lateral loculi. But their size and character vary a good deal in different 

 birds. They are largest in Rhea and the Ostrich. 



The gizzard is well developed in the Pigeon as in the Fowl and the Lamel- 

 lirostres s. Chenomorphae (Ducks, &c.). A short tube, which is always pale as com- 

 pared with the vascular proventriculus, connects that organ to the gizzard. The 

 pylorus is placed on the right side and close to the entrance of the proventri- 

 culus. The walls of the gizzard show two tendinous spots which lie one on the 



