xii PHYLUM CHORDATA 455 



varieties which differ from one another in size, proportions, 

 coloration, details in the arrangements of the feathers, and 

 in many points of internal anatomy. The following descrip- 

 tion refers especially to the Common Dovecot Pigeon. 



In the entire Bird (Fig. 257) the plump trunk appears to 

 be continued insensibly into the small, mobile head, with its 

 rounded brain-case and prominent beak formed of upper and 

 lower jaws covered by horny sheaths. The head, neck, and 

 trunk are invested in a close covering of feathers, all 

 directed backwards and overlapping one another. Posteriorly 

 the trunk gives origin to a number of outstanding feathers 

 which constitute what is ordinarily called the tail. From 

 the anterior region of the trunk spring the wings, also 

 covered with feathers, and, in the position of rest, folded 

 against the sides of the body. The legs spring from the 

 hinder end of the trunk, but, owing to 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. i' dg. /), 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\ used for lubricating or 

 " preening " the feathers. 



The wings show the three typical divisions of the fore- 

 limb upper arm, fore-arm, and hand, but the parts of the 

 hand are closely bound together by skin, and only three 



