Chap. II.] EXTERNAL CHARACTERS. 17 



central shaft (rachis) continuous with the quill, but differing from 

 it in being quadrate in section, grooved on its under side, and 

 filled with light white pith. On either side of the shaft are the 

 barbs, attached to it much in the same way as the teeth are set 

 on a comb, and, like them, flattened in a direction at right angles 

 to the axis. The barbs will be found to adhere together, so 

 that they cannot be separated without the application of some 

 gentle force, when they suddenly tear asunder. When the 

 continuity of the feather has thus been broken between any 

 two barbs, simple pressure of the two barbs together will not 

 readily mend it ; but if the lower or proximal part of the 

 broken vane be raised and hitched over the upper part, the 

 barbs will once more adhere together and the broken vane will 

 be mended. 



The cause of this will be evident if we cut off a little piece of 

 the vane, and, after soaking it for a few minutes in alcohol, 

 examine it under the microscope (low power). Each barb will 

 be seen to give rise to smaller barbs, or barbules, arranged on 

 either side of it. The distal barbules (those nearer the feather 

 tip) carry two or three hooks apiece ; the proximal barbules 

 (those nearer the quill) are simple and without hooks. When 

 the vane is perfect, the booklets on the distal barbules hook over 

 the proximal barbules ; and when we mend the broken vane, in 

 the way above described, we hitch the invisible hooks over a 

 series of invisible bars. 



In the filoplumes the vane is rudimentary, and the barbules 

 are provided with no hooks. 



There are seven apertures, three median and two pairs lateral. 

 The most anterior median aperture is the mouth, guarded by 

 horny mandibles, and provided with no teeth. The posterior 

 nares open by a common longitudinal slit in the roof of the 

 mouth, bounded by palatal folds. At the posterior end of this 

 slit are the openings of the Eustachian tubes. The tongue is 

 narrow, pointed, and horny. The oval glottis and the wide 

 gullet may be seen with difficulty at the back of the buccal 

 cavity. At the posterior end of the body there is a common 

 aperture (cloaca) for the exit of the faeces and the urino-genital 

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