The relation of coarse and microscopical ana tomy will be best seen by the following table : This may be taken as a tabular view of the typical structure of veins of larger size, and in the smaller vessels the same order of parts is represented, but with less distinctness and greater tenuity. There are certain regions, however, where the venous texture, both in quality and order, departs considerably from the typical arrangement : these will be noticed hereafter.
In the following observations, the several coats of the veins will be described in succes sion, beginning with the innermost.
Epithelium.— It is difficult (as has been ob served by Hen/e), in all cases, to make out a dis tinct epithelial layer lining the vascular cavity; but, as far as my observations go, it is more constant in arteries than in veins, and the epithelium is more perfectly formed in the former than in the latter. The anatomist will frequently make search in vain for epithelium on the inner surface of veins, and, when found, a perfect epithelial cell is less common than one imperfectly formed, the nucleus existing, but the cell-wall either partially or wholly absent.
The best method for examining the epithe lium is by scraping the inner surface of the vessel, and placing the material removed on a slip of glass; or by viewing the free edge of a valve under the microscope. Existing in its most perfect form the epithelium is of a dia mond or rhomboidal figure, containing a nucleus, large in proportion to the cell, of a granular character, and lenticular or oval form. The nucleus is distinct and well defined. The cell itself is clear, pale, and watery to an ex tent that it is impossible to portray in a wood cut. Henle states that the long axis of the rhom boid corresponds with that of the vessel in which it exists. In the figure 853. a), is re presented the most perfectly formed specimen I have seen from a vein ; the rhomboidal figure being very marked : it was obtained from the versa cava of a sheep. There is also seen (fig. 853. b), a more common form, where the cell and nucleus are both present, but the structure is less regular and more confused. The epithelium is of the pavement or scaly form, existing in a single layer : the cells are flat, and either have no cavity, the opposed sides being adherent, or the cavity is very minute. This is displayed in the drawing (fig. 853. c), where some cells are seen edge wise : in this position the dimensions of the nucleus are observed to bulge out the cell walls where it is placed, while the cell itself forms a comparatively narrow line, or is even reduced in appearance to a mere linear fila ment attached to the nucleus, sometimes at one extremity and sometimes at both.
But the condition in which I have most commonly found the epithelium is that re presented at d in the accompanying figure ; the nucleus, the essential part of the cell, being apparently all that is present, and re presenting the whole epithelial structure. These lenticular corpuscles, the nuclei, are scattered on the inner surface of the vein in pretty much the same position and form as if the cells were present, but the cell-walls are not sketched out, or the nuclei are only here and there partially invested by a cell. This appearance I have observed in the freshest specimens, and it may also be produced by keeping those where the cell is distinct for a short time, when the cell-wall liquefies, and becomes invisible. From the fact that the nuclei are in some instances pretty equally separated from each other, and hold about the same relative distances, whether the cell-wall he present or not, it would seem not impro bable that, where not visible, the cell might still exist in a state of imperfect and indistinct formation,—that the blasteina may be present in a mucilaginous condition, but of sufficient density to retain the nuclei in their proper relative position.
The nuclei may sometimes be seen thus arranged on the surface of a valve near its clear, thin edge, as is seen with unusual distinctness in fig. 865. A. These appear ances are generally destroyed by manipulation. When a valve is placed for examination, the nuclei are apt to float off its surface in the fluid with which the object is moistened, and arrange themselves along the free margin of the valve in the interval between its edge and the glasses.
Henle has represented, in his matchless work on general anatomy, the edge of a valve magnified, in which the nuclei of epithelium are arranged on a clear area, which rims the margin of the valve ; and this transparent, structureless boundary he describes as the epithelial cells, in which the nuclei, also visible, are embedded. I have seen well de veloped epithelium at the edge of a valve, but it did not present the appearance depicted by Henle, the cell being a mere line as repre sented in fig. 865. A. a, and scarcely forming a transparent edge. I have also occasionally seen a structureless rim to a valve, and as that was not changed by the washing and brushing off of the superimposed epithelium, I have given it a different interpretation: I have indeed considered it as a reduplication of the fenestrated membrane, existing there in a state of great tenuity and destitute of fibrous striations.