FOOD AND ITS UTILISATION loi 



its walls which are said to squeeze the juice out of the 

 arum leaves on which the bird feeds. It has thus taken on 

 a gizzard function, and it has become so large that the keel 

 of the breastbone has had to give way in part to make room 

 for it, and the clavicles are bent outward. 



" Pigeon's Milk." — Many people suppose that there is 

 something jocular about " pigeon's milk " ; others suppose 

 that they know all about it. But there is no joke and there 

 isnosatisfactory understanding of what happens. " Pigeon's 

 milk " is a milky fluid with cheese-like solid particles, and 

 it is produced in the crop of both sexes by a fatty degenera- 

 tion and discharge of many of the cells of the lining epithe- 

 lium. The milkiness is due to the fatty granules in the 

 disintegrating discharged cells ; the cheese-like particles 

 are minute groups of discharged cells which have not 

 degenerated so far. 



As every one knows, the parents regurgitate the " milk " 

 into the mouth of the young pigeon for some time after 

 hatching, and the fluid doubtless serves as a readily digestible 

 food until the young one is able to tackle more difficult 

 material. In other words, it serves in gastric education. 

 The regurgitation is mainly due to two special muscles 

 which rise from the bird's merrythought. It should be 

 carefully noticed that the lining of the crop is not glandular, 

 and that the pigeon's milk is not a secretion. It is a dis- 

 charge of lining cells which have undergone a pecuhar 

 transformation. The crop is an area for storage, not for 

 digestion ; but its worn and degenerate (we might almost 

 say moulted) lining cells have come to have a use in the 

 early nutrition of the young squabs. The same is said to 

 be true of some parrots. It may be suggested that the 

 discharge of the pigeon's milk should be brought into line 

 with the occasional moulting of the cuticular lining of the 

 gizzard, and some analogous phenomena. But it is a 

 desquamation and degeneration of cells, not a secretion or 

 the liberation of a secretion. 



At most seasons of the year it is possible to get some 

 " pigeon's milk " out of the crop, and perhaps there is an 



