478 



TEGUMENTARY ORGANS. 



that the frog is rudimentary. The horny 

 ecderon presents both tubuli and laminae. 



The excrescences on the inner surface of 

 the leg of the horse are identical with the sole 

 of the foot in structure consisting of a 

 horny mass penetrated by long papillae. 



The hollow horns of the Ruminantia are, to 

 all intents and purposes, Claws. The super- 

 ficial cellular ecderon (epidermis) is continued 

 upon them, and, when this is removed, we 

 come to a laminated fibrous horny mass, 

 which is formed and increased by apposition 

 from the subjacent process of the enderon, 

 supported by its bony axis a process of the 

 frontal bone The enderon has neither villi 

 nor lamellae, presenting only small irregular 

 ridges (Gurlt). 



The horn of the rhinoceros is commonly 

 said to be constituted by a mass of hairs which 

 have coalesced. However, it consists of an 

 aggregation of tubes, round which the horny 

 matter is arranged in concentric laminae, as in 

 the horny excrescence of the horse's leg; and 

 as there is no evidence of its having ever been 

 enclosed within a sac, it is more probable that 

 it belongs to the series of the claws and nails. 

 Glands, hairs, and feathers. The Hairs 

 and Spines of mammals, the Feathers of birds, 

 and the Integumentary Glands agree in one 

 essential point, that their development is 

 preceded by that of an involution of the 

 ecderon, within which they are formed, and by 

 which the former are, at first, entirely en- 

 closed. 



At an early period, the rudiments of the 

 hairs, and those of the cutaneous glands of a 

 foetal mammal, are indistinguishable. They 

 alike consist of solid processes of the 

 ecderon, consisting of a homogeneous matrix, 

 in which lie closely-set endoplasts, bounded 

 internally by a clear, narrow, transparent 

 " basement membrane," which at once sepa- 

 rates them from, and connects them with, the 

 enderon.* Externally these processes are con- 

 tinuous with the rete mucosum of the ecderon. 

 In the foetal lamb, in which I have carefully 

 traced the development of these processes, 

 they increase in size without change of struc- 

 ture, until, in the ordinary hairs, they have 

 attained a length of y^ inch ; for the vi- 

 brissae, that of -^ inch. Having reached 

 this length, it is seen that an accumulation 

 of the indifferent tissue of the enderon has 

 taken place around their coecal ends, which 

 gradually become pushed in, so that, from 

 being rounded, they appear truncated in 

 section, and present a bulb with a hemisphe- 

 rical involution, the rudiment of the papilla. 

 In the ordinary hair no special accumulation 

 of indifferent tissue takes place around the 

 body of the involution; but in the vibrissas, 

 which are ultimately to possess a thick outer 

 capsule, its foundation appears in this form, 

 and a capillary loop may be seen penetrating 

 the rudimentary papilla. 



In the furthest advanced vibrissae the 



* The further development of the glands will be 

 most conveniently considered, together with their 

 histological structure, below. 



tissue of the axis of the sac was converted 

 into horny cells, the rudiment of the " fenes- 

 trated " or of the inner, horny rootsheath. 

 Over the papilla the rudiment of the hair shaft 

 was indicated by a conical process, horny at 

 its apex and marked by radiating lines. 

 Finally, on each side of the neck of the sac 

 there was a bulging process, the centre of 

 which was occupied by a mass of fatty-looking 

 granules, the future sebaceous glands of the 

 hair. 



Hairs are not normally susceptible of inde- 

 finite growth, but have, like the teeth, a fixed 

 form to attain. This form is always that of 

 a more or less elongated spindle, inasmuch as 

 the hairs are sharp at their points, becoming 

 broader and thicker in the middle, and dimi- 

 nishing again to their proximal ends. When 

 fully formed, and ready to fall out, in fact, this 

 end of the hair is either pointed, or more or 

 less ragged and brush-like. 



As soon as the finishing process of any 

 hair begins the foundation of a new one is 

 laid by the development of a diverticulum of 

 the outer rootsheath towards its base, in 

 which a young hair is developed, in the man- 

 ner already described, and gradually pushes 

 out the old one. 



The varieties of form and appearance pre- 

 sented by the hairs of animals (for which see 

 the works of Hensinger, Eble, Busk, and 

 Quekett, cited at the end of this article) are 

 produced ; 1st, by the relative proportions of 

 the medullary and cortical substances, and the 

 arrangement of the former with respect to the 

 latter. Thus the peculiar appearance of 

 Rodent hairs is due to the disposition of 

 the medullary substance. 2nd, by the deve- 

 lopment of the cuticular layer, whence arise 

 the whorled scales of bat's hair the imbri- 

 cated plates of seal's hair, &c. ; 3rd, by the 

 shape of the shaft, which may be cylindrical, 

 as in ordinary hair of the head in man ; or 

 evenly flattened, as in the short curly hairs ; 

 or narrow and cylindrical below, and wide 

 and flattened above, as in the hairs of the deer 

 tribe. The spines of certain mammals, such 

 as Hystrix and Erinaceus, present some inter- 

 esting peculiarities of form ; offering, as they 

 do, a sort of transition between hairs and 

 feathers.* 



The porcupine's " quill," as it is called, is 

 a cylindrical tube which gradually diminishes 

 to a point above and below. At its apex the 

 cavity of the quill is simply conical, but lower 

 down its section becomes polygonal, and, the 

 angles of the polygon being prolonged, resem- 

 bles a four-rayed star. Still further towards 

 the root of the quill, each ray of the star 

 divides into two secondary rays, and then the 

 secondary rays subdivide into two tertiary 

 rays ; so that eventually the cavity of the spine 

 is a complicated star with four and twenty 

 branches. Below its middle, the quill dimi- 

 nishes in diameter, and at the same time the 

 complexity of its internal cavity likewise dis- 



* See Brocker (Reichert's Bericht, 1849), from 

 whom the account in the text is taken, though the 

 main points have been independently verified. 



