Bauhaus-Universität Weimar

The Cyclopaedia of Anatomy and Physiology, vol. 4: Pla [corr.: Ple] - Wri
Todd, Robert Bentley
50 i 
the wider will be the interval between the 
lines. When the section passes for any dis¬ 
tance in the plane of a lamina, no lines will 
present themselves on that space. And thus 
the appearance of a section of nacre is such 
as to have been aptly compared by Sir J. 
Herschel* to the surface of a smoothed deal 
board, in which the woody layers are cut per¬ 
pendicularly to their surface in one part, and 
nearly in their plane in another. Sir D. 
Brewster appears to suppose f that nacre con¬ 
sists of a multitude of layers of carbonate of 
lime alternating with animal membrane ; and 
that the presence of the grooved lines on the 
most highly-polished surface is due to the 
wearing away of the edges of the animal la¬ 
minae, whilst those of the hard calcareous la¬ 
minae stand out. If each line upon the na¬ 
creous surface, however, indicate a distinct 
layer of shell-substance, a very thin section of 
mother-of-pearl ought to contain many thou¬ 
sand laminae, in accordance with the number 
of lines upon its surface ; these being frequently 
no more than 1-7500th of an inch apart. But 
when the nacre is treated with dilute acid, so 
as to dissolve its calcareous portion, no such re¬ 
petition of membranous layers is to be found : 
on the contrary, if the piece of nacre be the 
product of one act of shell-formation, there is 
but a single layer of membrane. The mem¬ 
brane is usually found to present a more or 
less folded or plaited arrangement ; but this 
has generally been obviously disturbed by the 
disengagement of carbonic acid in the act of 
décalcification, which tends to unfold the 
plaits. There is one shell, however, — the 
well-known Haliotis splendens,—which affords 
us the opportunity of examining the plaits 
in situ, and thus presents a clear demonstra¬ 
tion of the real structure of nacre. This 
shell is for the most part made up of a series 
of plates of animal matter, resembling tortoise¬ 
shell in its aspect, alternating with thin layers 
of nacre ; and if a piece of it be submitted to 
the action of dilute acid, the calcareous portion 
of the nacreous layers being dissolved away, the 
plates of animal matter fall apart, each one 
carrying with it the membranous residuum of 
the layer of nacre that was applied to its inner 
surface. It will usually be found that the nacre- 
membrane covering some of these horny plates 
will remain in an undisturbed condition ; and 
their surfaces then exhibit their iridescent lustre, 
although all the calcareous matter has been re¬ 
moved from their structure. On looking at the 
surface with reflected light under a magnifying 
power of 75 diameters, it is seen to present a 
series of folds or plaits more or less regular; 
and the iridescent hues which these exhibit are 
often of the most gorgeous description. But 
if the membrane be extended with a pair 
of needles, these plaits are unfolded, and it 
covers a much larger surface than before ; 
and the iridescence is then completely de¬ 
stroyed. This experiment, then, demonstrates 
that the peculiar lineation of the surface of 
nacre (on which the iridescence undoubtedly 
* Edinb. Philos. Journ. vol. ii. 
f Loc. cit. 
depends, as first shown by Sir D. Brewster), 
is due, not to the outcropping of alternate 
layers of membranous and calcareous matter, 
but to the disposition of a single membranous 
layer in folds or plaits, which lie more or less 
obliquely to the general surface. 
There are several bivalve shells which pre¬ 
sent what may be termed a sub-nacreous struc¬ 
ture, their polished surfaces being covered 
with lines indicative of folds in the basement 
membrane ; but these folds are destitute of 
that regularity of arrangement which is neces¬ 
sary to produce the iridescent lustre. This is 
the case, for example, with most of the Pecti- 
nidæ, also with some of the Mytilaceæ, and 
with the common Oyster. Where there is no 
indication of a regular corrugation of the 
shell-membrane, there is not the least approach 
to the nacreous aspect ; and this is the case 
with the internal layer of by far the greater 
number of shells, the presence of nacre being 
exceptional, save in a small number of families. 
The membranous shell-substance, some 
form of which constitutes the internal layer 
of most bivalve shells, is occasionally traversed 
by tubes, which seem to commence from the 
inner surface of the shell, and to pass towards 
the exterior. These tubes vary in size from 
about the 1-20,000th of an inch, or even less, 
to about the l-2000th ; but their general 
diameter, in the shells in which they most 
abound, is about l-4000th of an inch. The 
direction and distribution of these tubes are 
extremely various in different genera. Thus, 
in Anomia Ephippium they are scantily dis¬ 
tributed in the internal nacreous lamina ; 
but in the yellow outer layer they are very 
abundant {fig. 415.), forming an irregular net¬ 
work, which spreads out in a plane parallel 
to the surface. In Cleidothœrus chamoides, on 
the other hand, the tubes are abundant in the 
internal layer of the nacreous lining, where 
they form an intricate but irregular net-work 
parallel to the internal surface of the shell ; 
and from this arise a series of straight tubes, 
which pass nearly at right angles with the 
surface, at a considerable distance from each 
other, through the external portion of the 
nacreous layer, towards the cellular structure 
which constitutes the exterior of the shell. 
This, however, they do not penetrate; stopping 
short as they approach it, just as the tubes of 
dentine cease at its plane of junction with the 
enamel. The diameter of the tubes is toler¬ 
ably uniform, even when they divaricate ; the 
trunk not being much larger than either of 
the branches. In other instances, however, 
no such net-work is formed, but the tubes 
run at a distance from each other, traversing 
the shelly layers obliquely, and are then 
usually of comparatively large size ; this is 
the case, for example, with some species of 
Area and Pectunculus. That these tubes are 
not mere channels or excavations in the shell- 
substance, is proved by the fact that they 
may be frequently seen very distinctly in the 
decalcified shell-membrane. They often pre¬ 
sent, in their beaded aspect, indications of a 
cellular origin, as if they had been formed 
o o


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