XVI. 22 



ALIMENTARY CANAL 



469 



though in owls these are regurgitated with fur or feathers, making 

 characteristic pellets. The crop of pigeons is also remarkable for the 

 milk it produces to nourish the young. There are special glands for 

 this purpose and they become active in the breeding-season under 

 the influence of a pituitary hormone, prolactin, which has been 



ur&vj 



Fig. 282. Diagrammatic section through cloaca of pigeon. 

 ap. external aperture; b.Fab. bursa Fabricii; ep. epidermis; m.sph. 

 sphincter muscle; muc. mucous glands; r. rectum; ur, & vd. papillae 

 for ureter and vas deferens (or oviduct). (After Clara, from Streseman.) 



crystallized and is probably protein in nature. Prolactin causes 

 regression of testes and ovaries and involution of secondary sexual 

 characters, but induces brooding behaviour in the female. Its action 

 is comparable with that of the galactogenic hormone of the mam- 

 malian pituitary. 



The duodenum and coiled intestine are of characteristic vertebrate 

 tvpe, relatively rather short, though somewhat longer in grain-eating 

 birds. The bile and pancreatic ducts usually open into the distal limb 

 of the duodenum; in pigeons the left bile-duct enters close to the 

 pylorus (Fig. 281). There is a peculiar pair of coeca at the junction of 

 rectum and intestine. The food enters these coeca, but it is not clear 

 what function they perform, possibly it is related to the absorption of 

 water. The arrangements of the cloaca are certainly concerned with 

 this end (Fig. 282). The rectum opens into a coprodaeum and this in 

 turn receives a urodaeum, which is the terminal portion of the urinary 

 and genital ducts. A final chamber, the proctodaeum, opens at the 

 anus. The urinary products are made solid by subtraction of water in 

 the urodaeum and the walls of the other chambers serve a similar 



