Page:Microscopicial researchers - Theodor Schwann - English Translation - 1947.pdf/101

Rh it is no longer possible to distinguish the two lamellae 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. 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 described 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. Lamellæ, 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 Räuschel (de Arter. et Venar. Structurâ) 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 lamellæ. 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 fœtal pig, large lamellæ 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 intimately 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 (Müller’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 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 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. The cells which are prolonged into cylinders con-