490 
Vertical section of the epithelium of the mouth, shew- 
ing its lamelle and the changes uf form which the 
particles successively undergo. 
a, superficial laminz, consisting of true scales ; 
b, c, particles in progress of flattening; d, deep 
layer of particles; a’, b’, c’, d’, separate particles 
in the several stages. Magnified 300 diameters. 
Fig. 279. 
A few scales detached from the surface of the uvula. 
Magnified 300 diameters, 
the homology of the mucous membranes with 
the skin. The term ‘ scales’ is only applicable 
to these particles in the last of the stages 
through which they pass. They first appear on 
the surface of the basement membrane as gra- 
nular dots, each of which soon becomes in- 
vested with a cell membrane. Both nucleus 
and cell increase in size up to a certain point, the 
cell being then more or less globular, and con- 
taining a material that appears transparent and 
almost entirely fluid. By this circumstance, 
chiefly, it is distinguished from the spheroidal 
form of particle, presently to be noticed. The 
cell now begins to flatten, loses its fluid con- 
tents, and is at the same time the seat of certain 
changes by which its chemical properties are 
modified. At length its opposite surfaces unite, 
except where the nucleus intervenes, and a 
lamella of extreme tenuity results, which being 
now arrived at the surface is loosened and 
shed. It appears to be by the continual pres- 
sure arising from the growth of newly-formed 
particles that the peculiar characters of this 
variety result. Accordingly, the scales are 
only found constituting the superficial layers of 
a series (fig. 278, aa’). Itis met with in those 
parts only where foreign pressure, or more pro- 
perly friction, has to be encountered. In such 
parts a thick coating of epithelium’is evidently 
desirable, and the hard and almost horny 
qualities which these particles at length assume 
where most to violence, admirably 
adapt them for their object. On such parts, 
moreover, cilia would not be needed, and it 
would even seem that this variety of epithelium 
when converted into true scales possesses 
MUCOUS MEMBRANE, 
neither ee substance nor vital power to 
develope and su these exquisite organs. 
The scaly epitietions is pease for the 
tenacity with which its icles adhere to one 
another, and to the su on which they rest. 
This adhesion is manifest at all the stage 
through which the particles pass. It is stronge 
between particles at the same ee than be 
tween those at different stages of growth, s 
that there is always a tendency to a separatior 
into successive lamine on maceration or othe 
wise. Hence have resulted the divisions 
the epidermis into two, three, or more layer 
and especially that remarkable fallacy of n 
garding the rete mucosum as a distinct structure 
How far this adhesion is owing to the presence 
of an intercellular substance in all instance 
it is difficult to decide; but it seems highly 
probable that, in the deepest layers, where th 
particles are small and rounded, such asubstanee 
must exist in considerable abundance, filling uj 
the interstices, and serving asa kind of blastem 
in which the nuclei (or cytoblasts) of fresh parti- 
cles originate. I have lately (Jan. 1842) ascer 
tained a very curious fact, giving evidence of 
adhesion. This is, that the delicate thread 
drawn out of the cutis when the cuticle | 
stripped from a gee of macerated skin, cor 
sist entirely of the epithelium of the sweat 
ducts, the particles of which are so intimatel 
united with one another, and with those of thi 
deeper layers of the epidermis, as to allow ¢ 
being thus dragged out of their tube of bas 
ment membrane, often for a length of an eigh 
of an inch. 4 
The scaly epithelium is subdivisible into tw 
forms, the regular and the irregular. In 
Jormer, the scales are united = to edge in 
regular manner, as in the skin of the Frog an 
other reptiles, and on many internal surfae 
especially in the lower animals. In this fo 
the particles do not become so thin as in the” 
other, and the superficial scales are cast off 
lamine consisting of a single series and 
uniform thickness. In the datier form, ° 
overlap one another without order, and presi 
no regular figure. This is the ordinary for 
and is that presented in the skin and oth 
parts of Mammalia and Birds. -= 
Of the prismatic variety.*—In this the 
ticles have the shape of small rods, dispe 
endwise on the basement membrane, in a sit 
layer, the thickness of which depends on 1 
length. These rods are united to one ano’ 
by their sides, which are flattened for that’ 
pose. They are, therefore, prisms and 
cylinders, as Henle terms them. are 
almost invariably of very unequal thickn 
different parts, being bulged somewhere - 
the middle by their nucleus, which is oval, 
its long axis parallel to that of the par 
Their deep or attached ity 6 also, u: 
tapers to a point, in order, probably, to a 
room for new particles to spring up in th 
tervals. This is more decidedly the case w 
they clothe a convex surface, (as hat 0 
* To this the very appropriate term ¢ 
been lately given by Professor Todd. 
