IGO ANATOMY OF VERTEBRATES. 



becomes softened and macerated, and prepared for the triturating 

 action of the gizzard and the solvent power of the gastric secretion. 



The change Avhich the food undergoes in the crop is well knoAvn 

 to bird-fanciers. If a Pigeon be allowed to swallow a great 

 quantity of peas, they will swell to such an extent as almost to 

 suffocate it. 



The time during which the food remains in the crop depends 

 upon its nature. In a common Fowl animal food will be detained 

 about eight hours, while half the quantity of vegetable substances 

 will remain from sixteen to twenty hours. Hunter made many 

 interesting observations on the crop of Pigeons, which takes on a 

 secreting function during the breeding season, for the purpose of 

 supplying the young pigeons in the callow state with a diet suit- 

 able to their tender condition.^ An abundant secretion of a 

 milky fluid of an ash-grey colour, which coagulates with acids 

 and forms curd, is poured out into the crop and mixed with the 

 macerating grains. This phenomenon is the nearest approach in the 

 class of Birds to the characteristic mammary function of a. higher 

 class ; and the analogy of the ^pigeon's milk' to the lacteal secretion 

 of the Mammalia has not escaped popular notice. In fig. 80, one 

 side of the crop, h, shows the ordinary structure of the parts, the 

 other, c, the state of the cavity during the period of rearing the 

 young. The secretion consists of proteine with oil, but contains 

 no suo^ar of milk nor fluid caseine. 



The canal continued from the ingluvies to the stomach is called 

 the lower a3Sophagus ; at its commencement it is narrower and 

 more vascular than that part which precedes the crop, but 

 gradually dilates into the first or glandular division of the 

 stomach, which is termed the ' ^voYentviQ.vlu^'' {ventriculus suc- 

 centuriatus, hulhus glandulosus, echinus, infundibulurn), figs. 78, 

 79, 80, c. 



The proventriculus of the Bird, like the spiral valve of the 

 Shark, is an alimentary surface packed into the smallest space : 

 in the latter the membrane is chylific, in the former chymific or 

 digestive : every follicle is, in fact, a portion of the peptic secret- 

 ing surface, Avith its gastric tubuli at right angles thereto ; the 

 surface being moulded to form either a simple or compound 

 cavity. 



In birds with a mde oesophagus, fig. 78, a, the commencement 

 of the proventriculus is not indicated by any change in the 

 direction or diameter of the tube, but only by its greater 

 vascularity, by the difference in the structure of the lining mem- 



' XGIV. p. 124, 



