CHAP. XIII.] DIGESTION IN BIRDS. 475 



In the absence of specific information on this point, it is impos- 

 sible to state definitely whether the liquid secreted by the crop sub- 

 serves other than purely mechanical functions. In any case, it cannot 

 be doubted that the latter are of the greatest importance. The 

 fact that grains of wheat, oats, or barley, which have sojourned 

 in the crop are not usually softened, renders it improbable that they 

 there undergo any great chemical change. On the other hand, there 

 can be no doubt that, as was appreciated by Spallanzani, the crop 

 acts as a kind of ' hopper,' only allowing grain to pass into the pro- 

 ventriculus little by little, as the contents of the latter (grains and 

 gastric juice) are admitted into the powerful corn-mill the gizzard 

 where they are pounded together and converted into a liquid paste 

 which is passed on to the small intestines. 



Before passing from this subject, attention must be drawn to the 

 well-known fact, studied in a special manner by John Hunter 1 and 

 Duvernoy 2 , but deserving of a histological and chemical research con- 

 ducted from the stand-point of our present knowledge, viz. that in the 

 pigeon, and doubtless other birds, the crop undergoes a change both in 

 the male and female bird, during the incubation of the young, and for 

 some weeks afterwards. Its mucous membrane becomes very vascular 

 and hypertrophied, presenting marked folds and wrinkles. A milky 

 liquid is poured out from the open mouths of the glands and accu- 

 mulates in the cavity of the crop. It is said that this milky liquid 

 can be regurgitated, and serves as the sole food of the young pigeons 

 during the first days of their life 1 . 



Function of It is in the proventriculus that are situated the 

 the proventri- glands which secrete the gastric juice. This fluid was 

 cuius and giz- first co n ecte ^ by Spallanzani and Reaumur, and after- 

 wards examined by Tiedemann and Gmelin. These 

 observers found the reaction of the juice acid, discovered that it con- 

 tained hydrochloric acid and that it possessed the property of curdling 

 milk. It appears certain that it is only in the gizzard that actual 

 action of the gastric juice on the food takes place, its sojourn in the 

 proventriculus being very short and its mechanical state opposing 

 itself to a proper digestive action. 



The function of the muscular division of the stomach (the giz- 

 zard) has been sufficiently explained in the above remarks as well as 

 in connection with the mechanical function of the crop. 



SECT. 5. DIGESTION IN THE HERBIVORA. 



The first thing which strikes us in comparing the organs of diges- 

 tion of herbivorous as contrasted with carnivorous and omnivorous 



1 John Hunter, ' On a secretion in the crop of breeding pigeons for the nourishment 

 of their young,' Observations on certain parts of the Animal (Economy, London, 2nd ed. 

 1792, pp. 235238. 



- Duvernoy, see his edition of Cuvier's Lemons d'Anatomie Comparee, 2me edit., 

 Paris, 1836. 



