EXTERNAL FEATURES 13 



closely allied to the odoriferous glands of some reptiles. 

 It is a subcutaneous mass of glandular tubules surrounded 

 by a capsule of connective tissue without smooth muscle- 

 fibres. It is made up of two equal lobes, entirely or partly 

 separate, united at the apex in a projecting nipple. Each 

 of the lobes is autonomous, that is to say, each has its own 

 supply of blood-vessels and nerves, and can secrete in- 

 dependently of the other. Each lobe has usually at least 

 one efferent duct opening by the nipple, but in goat- 

 sucker and hoopoe there is only a single duct for the whole 

 gland. The excretory nipple, invested by the delicate 

 skin, encloses the efferent canals of the lobes and the terminal 

 portions of their enveloping capsules. Its extremity may 

 be naked or furnished with plumules or exceptionally with 

 pennae, and there may be strands of smooth muscle-fibres. 



Three types of preen-gland are distinguished by 

 P. Paris (1906) : (i) With the two lobes coalesced, a strong 

 reservoir, a globular delicate nipple, without a terminal 

 tuft of down, e.g. in sparrow, blackbird, and rook ; (2) With 

 the two lobes clearly defined, a strong reservoir, a long 

 delicate nipple, and a feebly developed terminal tuft of 

 down, e.^. woodpecker ; (3) With distinct lobes, no reservoir, 

 a short truncate nipple with thick muscular walls, and a 

 strong terminal tuft of down, e.g. stork, heron, flamingo, 

 and cormorant. 



According to Paris, the importance of the preen-gland 

 as supplying a lubricant or varnish to the feathers has been 

 much exaggerated. Removal of the gland in starlings, 

 wild duck, and some other birds was not followed by any 

 change in the state of the plumage. Obstruction of the 

 canals, e.g. in the fowl, seems to make no difference. The 

 secretion is fatty, varied in colour, without excretory pro- 

 ducts, but with a characteristic odour. Paris did not 

 find that the secretion was of use in increasing the imper- 

 meabiUty of the feathers. It had no toxic action in the 

 birds he studied, and neither its removal nor its hyper- 

 trophy had any effect on the health of the bird. 



It may be suggested, however, that although the secretion 



