498 



TEGUMENTARY ORGANS. 



plast. In the human hair sac there are 

 usually only one or two laminae in this layer, 

 but in Rodents there are said to be many. 



If we examine a hair sac above the level of 

 the bulb, it will be clear that these inner root- 

 sheaths are not generated from the contiguous 

 surface of the external rootsheath, as would 

 at first seem probable. No transitional forms, 

 in fact, are visible in the direction of the 

 transverse diameter of the sac. Traced to- 

 wards the base of the sac, however, it is ob- 

 vious that opposite the lower portion of the 

 bulb the inner layers of the outer rootsheath 

 become metamorphosed into horny cells ; and 

 that of these cells, the inner are converted 

 into the imperforate layer, while the outer un- 

 dergo a more complete cornification, and lose 

 all trace of their primitive endoplasts. The 

 clefts which ultimately exist between these 

 cornified plates are not present in the young 

 state, but are the results of a secondary va- 

 cuolation. They have nothing to do with the 

 disappearance of the endoplasts ; for traces of 

 the latter may be observed in the centre of 

 horny plates, at whose edges the clefts are 

 commencing (fig. 315. F). It would appear, 

 therefore, that the rootsheaths grow like the 

 shaft of the hair itself, not by addition to their 

 surface, but by growth of their deep-seated 

 inner ends. 



Such is the composition of the growing hair; 

 but the completely formed hair (see 2. 

 Morphology') presents very great differences 

 in the minute structure of its inner termina- 

 tion. In the first place, the shaft runs out 

 into an irregularly conical mass, like a worn- 

 out painter's brush. It consists, at its ex- 

 tremity, entirely of cortical substance, and 

 the cornification runs in irregular lines into 

 the indifferent tissue, which occupies the 

 bottom of the hair sac and represents both 

 pulp and outer rootsheath. The inner root- 

 sheaths terminate above this point, in an irre- 

 gularly horny layer, which unites with, and is 

 in a manner reflected into, the cuticle of the 

 shaft, which ceases above its brush-like ex- 

 pansion. Finally, the outer rootsheath in the 

 immediate neighbourhood of the inner, is me- 

 tamorphosed into large horny cells, like those 

 of the cellular ecderon. The development 

 of these from the indifferent tissue of the outer 

 rootsheath, may be very clearly traced. The 

 periplast first becomes enlarged and marked 

 off into definite granular areae around each 

 endoplast, and the limits of each area are 

 metamorphosed into clear horny walls. The 

 cavity which these inclose enlarges, and the 

 endoplast, with its surrounding granular mat- 

 ter, remains attached to one wall, and then 

 eventually disappears, while the cavities en- 

 large, and their walls thicken into clear horny 

 " cells," which may eventually be detached 

 from one another. 



The whole process of the completion of 

 the root of a hair, then, is simply a return of 

 the diverticulum of the ecderon, the meta- 

 morphosis of whose elements, so long as the 

 hair was in course of formation, was guided 

 and determined into distinct forms along cer- 



tain fixed lines, to its general tendency to 

 undergo the ordinary cellular metamorphosis 

 over its whole surface. With this return to 

 its primitive tendencies, the increase of the 

 hair of course ceases, and sooner or later it 

 is pushed out and falls away. 



The spines of the Porcupine, of the Hedge- 

 hog, and of the Echidna*, present in their 

 histological,as in their morphological relations, 

 an interesting approximation to feathers. Ex- 

 ternally, they are coated by a cuticle, while 

 the principal mass of their walls consists, at 

 the ends, of a fibrous horny substance ; in the 

 middle, there is added to this a medullary 

 substance composed of polyhedral horny 

 cells. 



Fig. 317. 



Feathers of the neck of the common Fowl. 



A, free edge of pulp ; B, c, medulla and cortex ; 

 D, transverse section of cortex ; E, a barb, with 

 barbulc partly detached from pulp; F, cornified cell, 

 from rootsheath ; G, horny diaphragms in the 

 quill. 



The section of the shaft of a fully-formed 

 feather presents exactly these constituents 

 except the cuticle ; the centre is occupied by 

 medullary substance (fig. 317. B,), composed 



* See Brockcr (Reichert, Bericht. Mull. Archiv. 

 1849). 



