From the vena eava of the human subject I have obtained epithelium, differing far from the normal form. The specimen consisted of flat, irregular cells, with a small bright nucleus ; some cells distinct, others pale and ill defined ; some densely granular, others scarcely so at all.
In Birds (Loon — Colymbus septentrionalis), I have seen the epithelium diamond-shaped, with a large, flat, bright nucleus.
In Batrachia (Frog — Rana temporaria), the epithelium was large, pale, and irregular.
In Fish (Cod — Grains nzorrlina) the epi thelium was large, irregular, and granular.
Fenestrated membrane. — This structure, which Henle has called the "fenestrated," or " striated " membrane, is placed intermediately between the epithelium and the fibro-vascular elements of the vessels' walls, and, from this circumstance, as well as its physical proper ties, it bears a strong resemblance to the basement or limitary membranes on the skin, the serous and mucous surfaces.
This structure has been described and figured by Henle, in a manner which exactly coincides with my own observations. When a portion of the membrane is stripped off the inner surface of a vein (or artery, for it is the same in either), and examined under the microscope, it is found to consist of a thin, continuous sheet, of a pellucid, structureless membrane, to which are adherent some re ticulated fibres, having a longitudinal direc tion — hence it is " striated." It is also fre quently perforated with small holes, from which circumstance it is called " fenestrated." This homogeneous membrane has the remark able property of rolling itself up in the form of a scroll, somewhat like the elastic laminw of the cornea. According to Heide., it rolls itself in the longitudinal direction, but I have found the same tendency in the opposite course. The apertures, or fenestrm, seen on the membrane, all have more or less of a cir cular or oval form, and I have found that the number and extent of these perforations de pend on the manipulation it has undergone. Its physical properties are peculiar ; it is crisp and somewhat elastic, and its inclination to roll up in a scroll is so great, that it is never seen in a flat form. The elasticity, which it pos sesses, is the reason, as it seems to me, why the fenestrm are universally of a rounded form ; for when any lesion is effected in it, retraction occurs all round from the injury, and thus makes the point or line of lesion the axis of a circu!ar or oval aperture, as the case may be. In several specimens of this tissue,
in which no fenestrm at first existed, I have produced them to any extent by pricking or lacerating it with needles. The fenestrx in the figure were thus produced.
The longitudinal striations appear to me to be some fibres of the next tunic, — the in ternal longitudinal fibrous coat—accidentally adherent to the homogeneous membrane.* In the accompanying drawing (fig. 854.) is represented a portion of this membrane, from the jugular vein of the red-throated diver (Colynibus septentrionalis). It was of a pale pink tint, from contact with the blood in the vessel, and, seen by transmitted light in the mi croscope, had a slight yellow cast. It was marked by fine longitudinal threads on one surface, which projected at some spots beyond the torn edge of the membrane.
Purkinje and Rauschel consider this mem brane as similar to the middle coat of arteries. Valentin describes it as a peculiar structure less membrane. Henle considers it as a tran sition from epithelium to fibrous tissue, in which opinion he is supported by Schwann.
The fenestrated membrane has been found on various parts of the internal surface of the arteries and veins ; but the best specimens I have seen have been from the principal veins of large birds (goose, loon, gull), &c. At the free margin of valves I have occasionally seen, as noticed above, a clear, structureless rim, which I have imagined to be this membrane reduplicated, the membrane being extremely thin, and the interposd fibrous lamina not reaching quite to the margin.
It is said by IIenle and others only to exist in a few places, and certainly it is not always to be found, though it is not perhaps right to limit its existence to those spots only where it has been recognised, as its extreme tenuity and firm adhesion to the next coat might ac count for its not being generally seen.* The tissue in question is best seen by slit ting up a vein and pinning it out on cork. The inner surface is then scratched with a needle after it has been moistened. The fenestrated membrane retracts, and its edges may be ruffled up and a small portion removed by the points of very flue forceps, and thus ob tained for examination, which is best done without any superimposed glass, as that flattens the coils and folds, and it is impossible to unravel them.