480 



TEGUMEXTARY ORGANS. 



nails, which has already been described as 

 occurring in the spines of the porcupine. 



On the surface of the feather-pulp a 

 series of ridges are developed, running pretty 

 nearly parallel with one another from an an- 

 tero-posterior groove upon the upper surface, 

 which marks the position of the future shaft, 

 to a line parallel with that groove upon the 

 under surface or the process, which is called 

 the raphe. These ridges, therefore, bound as 

 many grooves which branch off from the 

 medio-dorsal groove, becoming gradually shal- 

 lower, to the raphe. These secondary 

 grooves, as they might be termed, how- 

 ever, are not themselves simple ; their walls, 

 the ridges, being again produced into short 

 parallel laminae, and therefore giving rise 

 to tertiary grooves, branching off from the 

 secondary ones. Now, the whole surface 

 of the matrix being covered by an ecderonic 

 layer in process of conversion into the cor- 

 tical and medullary substances of the fea- 

 ther, the primary groove becomes filled by the 

 end of the shaft ; the secondary grooves by 

 the terminal barbs, the tertiary grooves by their 

 barbules, while the processes appear to be 

 outgrowths from these. Were all this conical 

 horny cap to remain entire, the result would 

 be a very complex sort of porcupine's quill ; 

 instead of this, however, it breaks up along 

 the line of each ridge, and so we have a feather. 



The extremity of the feather being thus con- 

 stituted, how is its remaining length developed ? 

 According to Reichert, the whole pulp elon- 

 gates, and as fast as a portion of the feather 

 is completed, the corresponding segment of the 

 pulp dries up, constituting for the vane what 

 has been called the inner striated membrane 

 (e f ). However, I believe that this is not the 

 case, the inner striated membrane being, like 

 the outer, a mass of cornified cells detached 

 from the surface of the pulp, just as we shall 

 see the pith of the shaft to be, though this 

 has been also declared by Reichert to be dried- 

 up pulp. I believe that the growth of the 

 feather, on the other hand, resembles that of 

 the hairs and nails; viz. the extremity as it 

 is finished, is pushed up by the growth of the 

 base, the pulp only supplying materials from 

 its surface ; and I account for the inner stri- 

 ated membrane by supposing that a compara- 

 tively imperfect development of horny cell 

 membranes takes place from that surface of the 

 pulp which would otherwise be left bare, when 

 the terminal cone or plume of the feather is 

 pushed away. When the development of the 

 shaft has gone on in this manner for a longer 

 or shorter time, according to the length of 

 the feather, a change takes place. The pri- 

 mary groove, which has gradually widened with 

 the width of the shaft (to the exclusion of the 

 secondary grooves, which gradually shorten 

 and ultimately disappear) becoming shallower, 

 extends all round the pulp, and the formation 

 of medullary feather substance ceases, that of 

 cortical substance alone remaining. Thus is 

 the hollow quill formed, and its edges, not 

 quite closing above, leave the minute um- 

 bilical aperture by which the inner striated 



membrane is continued into the " pith "of the 

 quill. This pith is produced by the throwing 

 off' of successive transverse horny partitions 

 from the apex of the pulp, as the quill is 

 pushed beyond it: thus protecting itself from 

 the air admitted by the umbilical aperture, and 

 which is visible, occupying the chambers thus 

 formed (fig. 316. G). 



A full description of the various forms of 

 feathers is given in the article AVES in a 

 former portion of this work, to which the 

 reader is referred. 



There can be no question as to the relations 

 of the integumentary organs hitherto described 

 to the primary constituents of the integuments, 

 but it is different with regard to those cal- 

 cified tegumentary appendages, the scales of 

 Fishes, and the so culled " dermal" calcified 

 plates of Reptilia and Mammalia. One point 

 is quite certain with regard to these append- 

 ages, that they are not, like the calcified 

 shells of themollusca, the representatives of 

 the outer portion of the originally cellular 

 epidermis (are not therefore comparable to the 

 " cuticula" of a plant), inasmuch as the latter 

 may always, in their young state, be traced over 

 them. It is for this reason, I imagine, that 

 they are at present ordinarily called " dermal " 

 organs. A truly dermal or enderonic organ, 

 however, ought, if it continues to grow, to re- 

 tain the same characters as the enderon of 

 which it forms a part. It ought, therefore, to 

 have its protomorphic surface external and to 

 grow exogenously. Now, no scale or plate 

 of any fish, so far as I am aware, does this ; on 

 the other hand, it holds good of all, whether 

 Placoid, Ganoid, Cycloid or Ctenoid*, that 

 they commence by the occurrence of a calcfiic 

 deposit immediately beneath the cellular 

 ecderon, and that they increase by continual 

 addition to the inner surface of this primary 

 deposit. There are two ways in which we 

 may conceive that these scales and plates are 

 produced. Either they are a gradual calcifi- 

 cation of the whole enderon from without 

 inwards (which is the view taken by Leydig, 

 of the scales of Pol ypterus), in which case the 

 only tii-sue of the enderon capable of increase 

 (that of the protomorphic line) being arrested 

 by the calcareous deposit, the whole enderon 

 at these parts must cease to grow, which 

 would appear to be contrary to fact ; or the 

 scale corresponds with the cork-layer of the 

 vegetable integument, and like it, though 

 developed beneath the ordinary cellular epi- 

 dermis, is still a truly ecderonic structure. 



A great deal might be said for both these 

 views; and if in this place, I assume the latter 

 to be more correct, it is because I think we 

 must be guided by the homology of the scales 

 with certain other organs, where these rela- 

 tions are more definitely expressed. It may be 

 taken as certain, I think, that the scales, plates, 

 and spines of all fishes are homologous organs ; 

 nor as less so that the tegumentary spines of 

 the Plagiostomes are homologous with their 



* And I believe it will be found to be equally true 

 of the " dermal " bones of reptiles and mammals. 



