4i6 



TEGUMENTARV ORGANS 



these constituents except the cuticle ; the centre is occupied by 



medullary substance {fig. 317. B, a), composed of a coarsely granular 



horny substance excavated by polygonal cavities of about toVd- inch in 



diameter, frequently if not invariably containing air, which adds to 



the dark hue (by transmitted light) 



arising from the granular opacity 



of the horny matter. At its edges, 



this tissue passes into the cortical 



substance, which, in a transverse 



section {fig. 317. D) appears as a 



clear homogeneous or slightly 



granular mass, dotted over by 



minute apertures, about t^j-^o- in. 



in diameter, and tjo-Vf i"- apart. 



In a longitudinal section, on the 



other hand {fig- ^17. C, b), the 



general mass appears obscurely 



striated in a longitudinal direction ; 



and in the place of the circular 



apertures, we see elongated fissures, 

 somewhat narrowed at each ex- 

 tremity, whose transverse sections 

 constituted these apertures. The 

 pointed ends of the fissures were 

 continued by a line which could 

 frequently be traced into some other 

 fissure above or below, so that I 

 conceive the fissures are in reality 

 more or less complete canals. 



The quill of the feather is en- 

 tirely composed of cortical sub- 

 stance ; the barbs have the same 

 structure as the shaft ; the barbiiles 

 present both cortical and medullary 

 substances in a rudimentary condi- 

 tion. Each barbule in fact {fig- 

 317. E, e) exhibits along its axis a 



series of oval cavities, the remains of cells like those of the medulla, 

 while its lateral portions are composed of striated horny matter like 

 that of the cortex, and are produced into the curved and hooked 

 lateral processes {/). 



The polygonal cells of the medullary substance are produced from 



Fig. 317. — Feathers of the neck of the com- 

 mon Fowl. A, free edge of pulp ; E, c, 

 medulla and cortex ; D, transverse sec- 

 tion of cortex ; E, a barb, with barbule 

 partly detached from pulp ; F, cornified 

 cell, from rootsheath; G, horny dia- 

 phragms in the quill. 



