the relative thickness of the three elementary fibrous coats.
Specimens fur displaying the profile views of the walls of veins are best Obtained by slitting up the vessel, pinning it out upon cork, and suffering it to get dry. Sec tions are then to he made vertical to the surface, either parallel to, or at right angles with, the axis of the vessel. Thin shavings may then be removed with a very flat knife (and for this purpose, a Beer's cornea knife is the best), and when placed on a slip of glass, moistened, and covered with a square of thin glass, are ready for observation. The accom panying drawings were made from sections thus obtained.
When a longitudinal section of the in ternal tunic is examined with a high power, as in fig. 856. a, it is seen to consist of very fine yellow elastic tissue, which is peculiarly pale and indistinct, having mainly a longi tudinal direction, but being much interlaced and matted together, so that its longitudinal course is, in many situations, obscure. This coat is seen to be fine, and dense, and in strong contrast to all the other structures, from which it is separated by a distinct line of demarcation. When thus viewed, the fibres of this tunic are seen to present a succession of waves, not unlike those of white fibrous tissue, but finer and smaller : whether these undulations are from its own inherent pro Internal tunic of longitudinal fibres. — The internal tunic of longitudinal fibres is ex tremely thin, and occupies but a very small amount of the thickness of the vessels' walls. This is well seen in fig.855.a, which is a trans verse section of the subclavian vein of ,an ox, made vertical to the surface, and displaying perties, or whether they are produced by its adhesion to the next tunic, whose elastic con traction is greater than its own, thus throwing it into folds and waves, I am unable to say. The identity of these fibres with yellow elastic tissue is shown by the action of acetic acid, which does not destroy them, but renders them even more distinct, while the contiguous masses of white fibrous tissue are obliterated. When treated with this reagent the fibres of this coat lose their wavy character to a cer tain extent, and present intersecting undula tions. (Fig. 857. a.) When the internal coat
is seen in transverse section, it presents a granular, indistinct appearance, without fibres of any determinate direction : in some places it presents lines of a crumpled or corrugated as pect. In fig. 858. a, this may be seen, as also its distinctness from the next coat, from which it is separated at one part by a slight interval.
This coat appears to exist in all the veins, and in the smaller ones, and larger capillaries when treated with acetic acid, as hereafter to be described, its presence would seem to be indicated by the internal longitudinal nuclei, which are then displayed.
Middle coat of intermixed circular and lon gitudinal fibres. — This tunic occupies about one-fourth of the entire thickness of the wall of a vein. Its internal boundary is sharp and distinct, where this coat is in contact with the internal, but the outer boundary, where it gra dually merges into the external, is indefinite, and indeed artificial.
In fig. 856. the middle coat is represented at b, and is composed of intermixed fibres of longitudinal and transverse yellow elastic tissue embedded in a nidus of white fibre. As this figure represents a longitudinal section, the transverse fibres of the yellow element are seen cut across, and appear as small discs : on the other hand, the longitudinal fibres with which they are mingled, are seen in pro file, as interlacing and parallel rods : the former gradually diminish, and the latter, in the same proportion, increase, as viewed fur ther and further front the inner surface ; and that point where the discs entirely disappear and the longitudinal fibres alone remain may be considered as the external limit of the middle coat. This limit is very clearly seen in a specimen treated with acetic acid (fig. 857. b) ; when, as in fig. 858. b, the sec tion has been transverse, the discs and rods of yellow elastic tissue occupy a position and proportion the reverse of what has been de scribed. The discs are the most abundant on the outer, and the rods on the inner part of this coat, the former being seen in section, and the latter in profile.