414 TEGUMENTARY ORGANS 



is composed of an enderonic and of an ecderonic portion. The former 

 which is continuous with the subcutaneous tissues, when well 

 developed, consists externally of a network of fine elastic fibres,, 

 within which is a layer of homogeneous tissue containing endoplasts 

 which are more or less elongated transversely, and which form the 

 superficial layer of the enderon. Within this is a structureless layer, 

 the commencement of the ecderon, enclosed by which are the 

 representatives of the cellular ecderon, the so-called rootsheaths. 

 These are commonly described as two, the outer (ci) and the inner 

 {c, d) ; the latter again being composed of two structures, an external, 

 the fenestrated inner rootslieath of Henle, and an internal, which I 

 described in 1845, and which may be called the imperforate inner 

 rootslieath. The outer rootsheath, like the others, is thicker above than 

 below, thinning out where it joins the bulb at the bottom of the sac. 

 It consists entirely of tissue resembling that of the rate mucosum, and 

 needs no particular description. 



TYi^ fenestrated inner rootslieath lies in immediate contact with the 

 outer rootsheath. It is composed of more or less rounded or polygonal 

 flat plates, with faintly marked boundaries, united by their narrow 

 ends, and leaving spaces between their sides i^fig. 315. F). It is very 

 tough and resistant, both to mechanical and chemical action, and no^ 

 endoplasts can be seen in its elements. The imperforate rootsheath {a) 

 is composed of flat thin flexible plates not unlike those of the pre- 

 ceding layer ; but they present no intervals, their boundaries are 

 strongly marked, and in the centre of each there is a peculiar, elongated, 

 often more or less dumb-bell-shaped endoplast. 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 rootsheaths are not generated from the con- 

 tiguous 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 towards the base of the 

 sac, however, it is obvious 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 undergo a more complete cornifica- 

 tion, 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 vacuolation. 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 



