TEGUMENTARY ORGANS. 



479 



appears, the tertiary rays disappearing first, 

 anil then the secondary, &c., until at last the 

 cavity is circular as at the apex. The boun- 

 dary of the qnill cavity is immediately formed 

 by medullary substance ; but the cortical sub- 

 stance follows to a certain extent the con- 

 tour of the inner cavity, so that in a transverse 

 section of the middle of the quill the cortical 

 substance presents the same general outline as 

 the medullary, though its processes and insec- 

 tions are less marked. 



In the adult condition, the central cavity 

 is filled by an irregular horny mass, which 

 Reichert and Brocker regard as the dried-up 

 pulp, but which is probably, as in the feather 

 (vide infra), simply the last horny product of 

 the pulp, filling up the space which the latter 

 once occupied ; for it is certain that every por- 

 tion of the porcupine quill has, like every por- 

 tion of a feather, at one time constituted a 

 cap over the corresponding portion of its pulp. 

 The pulp, in fact, commences like that of a 

 feather, as a smooth conical process upon 

 which the apex of the quill is moulded. As 

 it grows, however, the pulp assumes an angular 

 form, and then, as that of a feather would 

 do, becomes produced into lamellae. By the 

 constant production of new elements at the 

 surface of these lamellae and their cornification, 

 the "quill" is produced, and retains internally 

 the impression of the mould on which it was 

 formed. Apart from the arrangement of the 

 lamella?, the principal difference from a feather 

 which the " quill" presents, is simply that it 

 does not, as it is formed, split up along the 

 lines of the lamellae of the pulp. 



In its main features, the process of deve- 

 lopment of feathers is identical with that of 

 hairs. A solid diverticulum of the ecderon 

 is first formed, within which the primary 

 change consists in the metamorphosis of cer- 

 tain median cells into a cone composed of 

 horny plates. There is thus formed, as in the 

 hair, an outer rootsheath, resembling and con- 

 tinuous with the retc mucosum, an inner root- 

 sheath, and a central papilla, the so-called 

 matrix of the feather (Jig. 306.). 



The horny rootsheath (Jig. BOG. c) attains 

 a very considerable thickness, and instead of 

 stopping short of the mouth of the sac, as in 

 the hair, its outer end is for a considerable 

 time pushed forwards by its basal growth 

 pan passu with that of the feather; so that 

 it eventually projects for a considerable dis- 

 tance beyond the surface. Finally, it opens and 

 allows of'the passage of the feather, which grows 

 through it, the horny layer ultimately forming 

 a true rootsheath around the quill. Like the 

 rootsheath of the hair, this structure consists 

 of two layers, an outer (c), denser and harder, 

 and an inner (d\ softer and more flexible. 

 The latter from being marked by the projec- 

 tin |p barbs of the young feather has been called 

 the striated sheath. Both layers, however, have 

 the same essential structure, being composed 

 of rounded or polygonal horny plates, whose 

 endoplasts are often distinctly retained even 

 in the outer layers. The histological meta- 

 morphosia of the feather will be described 



below, but the manner in which it acquires 

 its ultimate complex general figure requires 

 particular attention. Referring for further de- 



Fig. 306. 



Sectional vie/v of feather in its sac : Fowl, 

 e, barb ; /, pulp ; g, veual. 



tails to the article AVES, I may state here, that 

 every feather consists of the following parts : 

 the quill continuous with the shaft, or central 

 axis of the feather, which supports the hori- 

 zontally expanded vane, consisting of numerous 

 long, narrow, flattened lamina; ; the barbs or 

 primary rays, pointed at their extremities and 

 arranged with their edges upwards and down- 

 wards more or less perpendicularly on the 

 shaft. Arranged in a similar manner on the 

 barbs, are the barbules, which therefore are 

 disposed more or less parallel to the shaft ; 

 from the sides of these, lastly, project short, 

 toothed, curved, interlocking pn>ccsses. 



All parts of the feather are solid, except the 

 quill, which is hollow and occupied only by a 

 dry shrivelled mass, the pith, in its upper part, 

 while below, during life, it receives the pulp. 

 Superiorly, on the under side, where the quill 

 joins the shaft, there is a small aperture, which 

 communicates with the interior, with a short 

 canal in the shaft, and with a groove which 

 runs along its under surface. 



It may be well to remember that the apex 

 of a barbule resembles in structure one of its 

 own processes ; that of a barb, one of its bar- 

 bules ; that of the shaft, one of its barbs. 



The development of this complicated on;an 

 from its matrix or pulp takes place very 

 simply, by a sort of exaggeration of the com- 

 bination of hair development with that of the 



