ORGANS OF SECRETION. 603 



crease their internal cavity, develope from their vascular and 

 thickened periphery innumerable rudimentary tubuli, the prox- 

 imal cervix becomes contracted and extended to form the duct, 

 and from the parietes of the duct a small diverticulum pro- 

 trudes to constitute the future gall-bladder. In the substance 

 of the vascular enveloping blastema, the minute plumose 

 ramifications of the tubuli, of which the adult organ is chiefly 

 composed, are observed compactly aggregated together, and 

 apparently as yet without internal cavities, and they are 

 destitute of the terminal vesicular enlargements seen at the 

 closed ends of the pancreatic and salivary tubuli. The liver 

 in birds is still relatively large, variable in form and in the 

 number of its external divisions, with the left lobe generally 

 smaller than the right, and a distinct middle lobulus Spigelii. 

 There is still a separate hepatic duct which passes into the 

 duodenum above the entrance of the cystic, one or more 

 hepato-cystic ducts end in the fundus of the gall-bladder, 

 and the separate hepatic and cystic ducts open into the in- 

 testine at a considerable distance below the pyloric orifice of 

 the stomach. The gall-bladder is often wanting in this 

 class. 



The pancreas still opens as in reptiles by several ducts, 

 which penetrate the duodenum between the openings of the 

 hepatic and cystic ducts. The small spleen of birds is some- 

 times divided into separate lobes, as in some fishes, serpents, 

 and cetaceous mammalia. The thymus gland is a transient 

 organ in birds, as it is in most mammalia, and it makes its 

 appearance late in the progress of development in these 

 classes, as it appears late in ascending through the classes of 

 the animal kingdom. Two small thyroid glands are recog- 

 nizable in many birds near the inferior larynx ; and most 

 birds, especially the aquatic species, present over the dorsum 

 of the coccyx, two contiguous pyriform oil glands, composed 

 of straight parallel tubuli ; which open on the skin by several 

 papillar orifices, and afford the unctuous secretion with which 

 the feathers are dressed to render the plumage impermeable 

 to water. In most birds the rudiment of Cowper's glands 

 is observed as a single or bifid follicle, bursa Fabricii, opening 

 into the median dorsal part of the cloaca, between and behind 

 the openings of the two ureters. 



All the ordinary secretions of birds, from the high tempe- 



