EPITHELIUM. 



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it is no longer possible to distinguish the two lamellae of the cell- 

 membrane. It often occurs that the tabular epithelial eel Is 

 are not regularly hexagonal, but represent flat elongated stripes, 

 a fact which has been observed by Ilenle in the epithelium of 

 the vessels. 1 The cells which are prolonged into cylinders con- 



1 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. Lamellae, 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. Structure) 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 foetus are examined. I 

 obtained by scraping, both from the larger veins and heart of a foetal pig, large 

 lamella; 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 Ilenle's very complete 

 treatise (Midler's Arehiv, 1838, Heft 1). In addition to the parts mentioned by 

 Ilenle, 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 foetal 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 tcssclated (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 Ilenle. 



