4 PHILOSOPHY OF ZOOLOGY. 
When the feather first pushes itself beyond the surface 
of the skin, it has the appearance of a thorn. If it be 
opened in this state, it may be observed to cousist of a ~ 
sheath and medulla. 
The sheath is in the form of a tube, similar in its compo- 
sition to the quill ; open at the base, where it is inserted in 
_ the skin; but closed at the other extremity, where its apex 
possesses considerable hardness. The use of this tunic is 
to defend the central parts, while soft and tender. 
The medulia consists of a gelatinous central cord, con- 
taining bloodvessels, and covered with a dark glairy fluid. 
It extends from the apex of the sheath, to its insertion in 
the cellular web. It may be considered as the organ which 
forms the different parts of the feather. f 
The glairy fluid appears densest towards the apex, and, 
on the outer side, two white threads may be perceived close 
to each other, which have been formed in it. These 
threads gradually mcrease im size, and finally coalesce, 
leaving, however, on their inner surface, a small groove, 
more or less obvious, and which is permanent. In the re- 
maining part of the circle of this fluid, surrounding the 
central cord, the web of the feathers is formed. Each 
barb appears with one extremity attached to one of the la- 
teral edges of the shaft, and the other, pomting upwards 
and inwards, assists im formmg a sort of tube of barbs 
round the central cord. The whole is still restramed by 
the sheath, which frequently bears the impression, on its 
inner surface, of the marks of the barbs. Although, at 
this time, both the shaft and the barbs may be perceived in 
a state fit for exclusion, towards the extremity, yet, near 
the base, the glairy fluid scarcely exhibits a trace of the 
rudiments of the remaiming part of the feather. 
In proportion as this fluid, at the extremity, changes in- 
to the shaft and barbs, the central cord withers, and grows 
3 
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