EPITHELIUM. 79 
itis no longer possible to distinguish the two lamellee of the cell- 
membrane. It often occurs that the tabular epithelial cells 
are not regularly hexagonal, but represent flat elongated stripes, 
a fact which has been observed by Henle in the epithelium of 
the vessels." The cells which are prolonged into cylinders con- 
' During several years past I have occasionally observed an innermost apparently 
structureless layer in different parts of the vessels, and as the elastic fibres of the 
middle coat of arteries become gradually more and more minute towards the interior 
of the vessel, and at length are scarcely perceptible, I regarded the layer above de- 
scribed as analogous to the middle arterial coat, in every respect but the possibility 
of discovering fibres in it. I explained certain scattered spots which occurred in 
it, by analogy with the middle and external coats of vessels. Lamelle, for instance, 
were occasionally present, in which the elastic fibres had coalesced more or less 
intimately, and only a trace of a fibrous arrangement remained. In such instances 
there is seen a table composed of elastic tissue, perforated at different spots; I 
regarded those spots as openings which might perhaps be filled with some foreign 
substance. Purkinje and Rauschel (de Arter. et Venar. Structura) acknowledged 
the accordance of this membrane with the middle arterial coat, but distinguished it 
as a separate layer. Valentin denied that accordance, and described it as a peculiar 
structureless membrane. Henle was the first to explain its true relations. By his 
mode of scraping the internal surface of the vessels he obtained scales, which, 
from our present more accurate knowledge, we now recognise as epithelium. They 
were sometimes converted into lamella. There cannot in fact be a doubt about the 
correctness of this explanation, when the vessels of the fwetus are examined. I 
obtained by scraping, both from the larger veins and heart of a feetal pig, large 
lamellae of the most beautiful epithelium, consisting of flat stripes, which were nearly 
as long again as broad, and contained a very distinct and, in proportion to the size 
of the scales, large nucleus, with one or two nucleoli. I could not succeed so well 
in the few attempts which I made on arteries; probably the scales separate more 
readily from one another in them, and can then no longer be distinguished from the 
primitive cells of the elastic coat. The cells probably coalesce more or less in- 
timately at a subsequent period, so as to form what is then a partially structureless 
layer, and the nuclei also disappear in part. I now conjecture that the above- 
described spots upon the inner coat may probably be persistent nuclei; I have not, 
however, made any new investigation upon the subject. With respect to the situation 
in which the one or other form of epithelium occurs, I refer to Henle’s very complete 
treatise (Miiller’s Archiv, 1838, Heft 1). In addition to the parts mentioned by 
Henle, I have found epithelium upon the internal surface of the amnion in the foetus 
of mammalia and man, where the hexagonal scales were very large and beautiful, 
enclosing a very distinct nucleus and nucleolus. Amongst those in the feetal pig 
were some larger round cells, furnished with a larger nucleus without a nucleolus. 
The inner surface of the portion of the allantois projecting from the chorion in the 
same foetus was also lined with tesselated (tabular, scaly) epithelium consisting of 
small scales. The external surface of the chorion was formed of cylindrical cells 
closely packed together, and provided with a nucleus, being similar to the epithelial 
cylinders of the intestinal mucous membrane discovered by Henle. 
