140 EDWARD SCHAFER. 
primitive sheath of the nerve or sheath of Schwann; whilst 
enveloping all are the numerous laminz composing the neuri- 
lemma. There is, moreover, altogether within the latter, a 
certain amount of finely filamentous connective tissue, which 
has long been known and described (Sharpey), in which the 
nerve-fibre lies imbedded. According to Key and Retzius 
this fibrillar layer (which they name the “ endoneurium ”’) 
is bounded externally, next to the neurilemma, and perhaps 
also internally, next to the sheath of Schwann, by a delicate 
stratum of flattened cells. 
They further show, and it is not difficult to confirm the 
observation, that the lamellee which compose the neurilemma 
(perineurium) agree in structure with the coats of the Paci- 
nian, each lamella consisting of an inner and outer bounding 
layer of flattened cells enclosing fibres between them, the only 
difference being that in the case of the neurilemma the inter- 
stitial fluid is either absent or inconsiderable in amount. 
Moreover, the number of layers is far greater in the capsular 
envelope of the Pacinians. 
Tracing now the continuity of these various parts of the 
entering nerve with the parts of the Pacinian corpuscle, we 
tind, in the first place, the axis-cylinder of the nerve becoming 
directly continuous with the central fibre of the corpuscle 
(fig. 2). The medullary sheath, on the other hand, termi- 
nates, under ordinary circumstances, as soon as the nerve 
enters the core ; more rarely, as we have seen, it continues 
to surround the axis-cylinder in its passage through the core. 
It certainly does not expand to form the core, as has been 
sometimes supposed, for in the first place the core is not 
blackened by osmic acid; and in the second place, when 
the axial fibre retains its medullary sheath, this exhibits an 
entirely different appearance from the surrounding core, 
from which it is distinctly marked off. To take next the 
primitive sheath or sheath of Schwann. This is more or 
less closely applied to the medullary sheath of the nerve 
just before it reaches the core, but then (fig. 2, p s) appears 
to open out and to pass to the exterior of the core, running 
at first apparently between the outer and inner parts of the 
core, but at length becoming lost to view. So that the main 
part, at least, of the core, since it lies within the prolonga- 
tion of the sheath of Schwann (so far as this can be traced) 
between it and the central fibre, corresponds with the deli- 
cate protoplasmic layer which lies between the sheath of 
Schwann and the medullary sheath of a nerve-fibre. The 
outermost portion of the core, on the other hand, is distinctly 
continuous with the fine connective tissue in which the 
