380 /OOLOGY sect. 



the thick covering of feathers, only the feet are to be seen in the 

 living Bird, each covered with scales and terminating in four digits 

 (dg. 1' — dg. J/!), three directed forwards and one backwards. 



In order to make a fair comparison of the outer form with that 

 of other Craniate types, it is necessary to remove the feathers. When 

 this is done, the Bird is seen to have a long, cylindrical, and very 

 mobile neck, sharply separated both from head and trunk. The 

 true tail is a short, conical projection of the trunk, known as the 

 uropygium, and giving origin to the group of large feathers {ret) to 

 which the word " tail "is usually applied. On the dorsal surface 

 of the uropygium is a papilla bearing on its summit the opening 

 of a large gland, the oil-gland (o.gl.), the secretion of which is 

 used for lubricating or " preening " the feathers. 



The wings show the three typical divisions of the fore-limb, 

 upper arm, fore-avm, and hand, but the parts of the hand are 

 closely bound together by skin, and only three imperfectly-marked 

 digits, the second {dg. 3) much larger than the first {dg. 1) and 

 third {dg. 3), can be distinguished. In the position of rest the 

 three divisions of the wing are bent upon one another in the form 

 of a Z ; during flight they are straightened out and extended so 

 that the axis of the entire wing is at right angles to that of the 

 trunk. On the anterior or preaxial border of the limb a fold of 

 skin stietches between the upper arm and the fore-arm ; this 

 is the alar memhrane or pre-patagiuin {jjr.pigm.). A similar but 

 much smaller fold extends, postaxially, between the proximal 

 portion of the upper arm and the trunk ; this is the post-patagium 

 {pt.ptgni.). 



In the hind-limb the short thigh is closely bound to the trunk, 

 not standing well out as in a Reptile, but directed downwards and 

 forwards ; the long shank extends from the knee downwards and 

 backwards ; and the foot is clearly divisible into a proximal portion, 

 the tarso-metatarsus {is. mtts.), and four digits, of which one, the 

 hallux {dg. 1'), is directed backwards, the others, the 2nd, 3rd, and 

 4th of the typical foot, forwards. The entire hind-limb is in a plane 

 parallel with the sagittal plane of the trunk. 



The mouth is terminal, and is guarded by the elongated upper 

 and lower beaks ; it has, therefore, a very wide gape. On each 

 side of the base of tlie upper beak is a swollen area of soft skin, 

 the eefc {cr.), surrounding the nostril {na.), which has thus a remark- 

 ably backward position. The eyes are very large, and each is 

 guarded by an upper and lower eyelid and a transparent nicti- 

 tating membrane {net. m.). A short distance behind the eye is the 

 a^ulitory aperture {cm. ap.), concealed by feathers in the entire 

 Bird, and leading into a short external auditory meatus, closed 

 below by the tympanic membrane. The anus or cloacal aperture 

 {a7i.) is a large, transversely-elongated aperture placed on the 

 ventral surface at the junction of the uropygium with the trunk. 



