240 ANATOMY OF VERTEBRATES. 



disappears from the summit of the medulla, aud ouly the mem- 

 branous funnel-shaped caps remain, Avliioh are }>rotruded from the 

 theca and the centre of the new-formed barbs, and fall off as these 

 expand. The theca which incloses the whole is of a firm texture 

 where the new-moulded barbs are yet pulpy and tender, but it 

 becomes thinner as tliese acquire consistency, and, lastly, dries and 

 crumbles away after it has been exposed to the action of the atmo- 

 sphere. The bulb itself, when examined in a half-formed quill- 

 feather, is composed of two parts, corresponding to the external and 

 internal aspects of the feather. The internal part represents a semi- 

 cylinder or case, inclosing the external part, which is of a conical 

 form ; the latter extends from the base of the bulb, and gradually 

 diminishes to a point where the shaft is completed and the barbs 

 begin to expand. Its office is to deposit the pith within the shaft, 

 and it is absorbed in proportion as this is effected. The internal 

 part or case also commences at the base of the bulb, and adheres 

 closely to the cone, with which, indeed, its substance is continuous ; 

 it increases in thickness as the cone diminishes, its margins are 

 beautifully scolloped or crenate, and the crenations are lodged in 

 the interspaces of the oblique laminae or moulds, and deposit in 

 them the material of the vane. The horny sides of the shaft are 

 lodged and formed in the grooves between the external and 

 internal parts of the bulb, and correspond in degree of formation 

 to the depths of those grooves ; and being progressively brought 

 into contact from above downwards, the shaft is thus completed, 

 leaving the longitudinal line at the internal side. When all the 

 grooves (wherein are formed the barbs, and the portion of the 

 shaft which carries them) are filled by the horny matter, and the 

 barbed part of the feather is finished, this horny matter lastly 

 expands uniformly around the medulla, and forms the quill of the 

 feather. 



When tlie quill of the feather has acquired the due consistence, 

 the internal medulla becomes dried up, and is resolved, as before, 

 into membranous cones arranged one upon the other ; but these 

 latter never pass out, for the quill, which is now hardened 

 and closed by the shaft at the extremity opposite to the lower 

 umbilicus, will not permit their egress ; they remain, therefore, 

 inclosed, and constitute the liglit dry pith which is found in the 

 interior of the quill. The last remains of the bulb are seen in the 

 ligament Avhich passes from the pith through the lower opening of 

 the quill and attaches it to the skin. 



There is a close analogy between the formation of a feather 

 and that of a tooth ; but a tooth may take years to be perfected, 



