ODONTOTORMjE— OIL-GLAND 653 



ODONTOTOEM^, see Odontornithes. 



(ESOPHAGUS (Greek ola-o(^dyos), so named by Aristotle, the 

 gullet or " swallow " of plain English {cf. Digestive System, page 

 136), the part of the alimentary canal from the Larynx (page 

 512) to the Stomach. It passes down the right and dorsal side of 

 the Trachea, with which and other adjoining parts it is connected 

 by loose tissue, entering the thoracic cavity dorsally from the 

 Bronchi (page 5S), and when not distended it forms numerous 

 longitudinal folds owing to the yielding nature of the tunica mucosa 

 (page 137). Deglutition is aided by simple mucous glands, but in 

 many birds the middle portion of the (Esophagus forms a per- 

 manent dilatation, the Crop (page 113), to the outer sui'face of 

 which thin but broad bands of striped or voluntary muscle are 

 generally attached. These may arise from the Furcula (page 296) 

 as in Pigeons, or from the skin of the neck as in the Gallinse, and 

 their action assists the conveyance of the food from the crop to the 

 stomach. During this process, especially if only little and dry food 

 be left, Birds, Parrots for instance, may be occasionally observed to 

 stretch their neck and gape widely with their mouth. 



OIL-BIRD, see Guacharo. 



OIL-GLAND (glandula uropygialis), in Birds the only cutaneous 

 gland except some small organs in the external ear-passages. Con- 

 sisting of two symmetrical portions, more or less united posteriorly 

 in shape of a heart — since each half is broad and rounded in front 

 and pointed behind — it is seated upon the levator muscles at the 

 root of the tail. Internally it is formed of numerous secretory 

 tubules which gradually unite in a common cavity opening on the 

 surface through a variable number of orifices — there being from 3 to 

 5 of them in many Water-birds, though only one to each half in 

 Anseres — frequently prolonged in form of a nipple and occasionally 

 united in a single tube, the double origin of which is, however, 

 shewn by a median septum. In the Hoopoe alone, according to 

 Nitzsch, there is but one orifice to the common cavity, wherein the 

 stinking secretion of the gland, for which the female during incuba- 

 tion and the young while they stay in the nest are notorious, is 

 stored. The whole structure, which is surrounded by connective 

 tissue and unstriped muscular fibres, is innervated by the first 

 caudo-spinal nerves, and its blood-supply is in connexion with the 

 caudal arteries and veins. 



In the majority of Birds this gland is well developed, being 

 largest in those of aquatic habit, and especially in the Tubinares 

 and Steganopodes, as well as in Pandion (Osprey) ; but it is also 

 large in Steatornis (Guacharo). It exists, though hardly in a 

 functional condition, in certain Pigeons (Ptilopus), Cacatua cristata 

 (Cockatoo), and most Caprimulgi (Nightjar), while in other 



