166 EDWARD B. POULTON. 
light upon the corresponding sheath as it is described in other 
mammals. As in the latter, the inner root-sheath surrounds 
that part of the hair which is enclosed in the follicle, but grow- 
ing less rapidly it does not extend to the neck through which 
the hair protrudes; hence we do not find it at all in sections 
of the upper part of the follicle (figs. 17 and 18). It is far 
thicker and more important in the hairs of the young animal, 
and especially in those of the tail (figs. 21 and 22,7.7.s.). In 
structure it consists of a network of corneous fibres enclosing 
fusiform meshes with a longitudinal direction, through which 
the outer root-sheath can be seen (fig. 24). Hence in certain 
cases Henle’s original description of his inner root-sheath as a 
fenestrated membrane is certainly true of Ornithorhynchus. 
Henle’s account is also followed by Mertsching (‘ Archiv f. 
Mikr. Anat.,’ 1888, p. 32), who shows in pl. v, fig. 8, that this 
sheath in the human hair possesses a fenestrated structure. In 
transverse section the corneous fibres of Ornithorhynchus are 
seen to be polyhedral and irregular in outline (figs. 19, 21, 22, 
i. 7. 8.), and if the section be taken at some little distance above 
the bulb, distinct spaces appear between them (figs. 19 and 21). 
Round the small hairs, on the other hand, only a uniform layer 
of the proportions usually found in Mammalia could be detected 
(fig. 19). In some of the longitudinal sections of larger hairs I 
could make out a thin internal layer exhibiting a serrated 
edge, with teeth the reverse of those on the hair cuticle. This 
evidently represents the so-called cuticle of the inner root- 
sheath, and it is probably shown in section in fig. 21. Well above 
the bulb (fig. 21) this thin internal lamina was the only trace of a 
separation of the sheath into layers, and even this could be 
detected only occasionally ; and I gained the impression that it 
is not a distinct and definite layer, but merely the condensation, 
as it were, of the innermost part of the inner root-sheath upon 
the exterior of the hair and the moulding of its surface by con- 
tact with the cuticle of the latter. But at a lower level, just 
above the bulb, there is seen what I believe to be the homologue 
of Huxley’sand Henle’s layers. Thus in fig. 22 the fibres of the 
inner zone have not become corneous and take the stain readily, 
