94 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 consist of a 

 sheath and medulla. 



The slwatli 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 medulla 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. 



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 increase in 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, pointing upwards 

 and inwards, assists in forming a sort of tube of barbs 

 round the central cord. The whole is still restrained 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 remaining 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 



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