When viewed with a high magnifying power the divided extremities of the rods of yellow fibrous tissue form undulating, and somewhat imbricated lines of discs, while the white fibrous tissue, cut across, shows slight, indis tinct, wavy indications. (Fig. 858. c.) Seen with a low power, the ends of the yellow fibrous tissue appear as series of dark undulating dots on a field of white.
Minute veins. — When very small veins, a few removes from capillaries, are examined microscopically, they merely present faint stri ations in the longitudinal direction. When, however, these vessels are treated with acetic acid, nuclei and fibres are distinctly displayed, whose long axis may be assumed to indicate the direction of the tunic (or set of fibres) which they represent, in which they are em bedded, and of which they form a part.
Henle was, I believe, the first to point out this mode of textural dissection of vessels, and nothing can be more satisfactory than the analysis that it makes. By this means small vessels alone can be examined, but the scru tiny may be carried up to those the third of a line in diameter, which are sufficiently trans parent, when entire, to exhibit the nuclei. Small vessels may be conveniently obtained from the pia mater and mesentery ; they are there free from other structures, and their form is not interfered with.
I have found, however, that it is in the pia mater somewhat difficult to trace the small veins; the arteries about them are more de finite and conspicuous, and less injured by manipulation, and generally catch the atter. don of the observer. It has been more con venient to obtain isolated cerebral veins from the surface of the ventricles of the brain ; the small veins on the corpus striatum may be raised and torn away with the points of fine forceps, and sufficient capillaries will generally he found attached to their extremities to ex hibit the structure of these vessels of all size. The accompanying figures were made from specimens thus obtained, about whose venous character there could be no mistake.
Small veins vary in structure in different regions, and according to their size ; some approach the structure of arteries *, whilst others are far removed from them in com position.
Venous capillaries do not, that I am aware of, differ from arterial. They consist of tubes of homogeneous membrane, studded here and there with nuclei of a more or less oval form, and placed generally with their long axis cor responding with that of the vessel. In passing to larger veins the change in structure of the vessel (its increase and character) depends upon the region from which it is obtained, and whether or not it is endowed with muscle cells.
In the cerebral veins, which have no muscle, in passing from small capillaries to larger vessels, all that is observed is the superaddi tion of a tunic of areolar tissue, surrounding the lining membrane ; and when this tisste is treated with acetic acid, all that is observable is a series of nuclei, oval, irregular, and longi tudinal, embedded in the substance of the parietes, which have a perfect resemblance to the external, or cellular coat of small arteries. (Fig. 859. A.) In none of the microscopical cerebral veins have I been able to discover any nuclear or yellow fibres. In veins of the A to of an inch the vascular wall has been composed of a tissue rendered trans parent by acetic acid, and displaying oval and irregular nuclei having a longitudinal course, without any admixture of elastic fibre. The most internal of the nuclei appeared elongated, hut there was no appearance of fibres upon the internal membrane, so as to produce a striated membrane.
In those veins which are endowed with muscular tissue, small club-shaped nuclei are observed placed externally to the lining mem• brave, and transversely as it regards the ves sel ; these show themselves very distinctly on the addition of acetic acid, and are character istic of the "muscular fibre-cells" of KEdliker. Such may be seen in the small veins of the mesentery, which are more easily examined than any other veins of this class. In these vessels, two or three removes from capillaries, the muscular nuclei make their appearance, at first, few in number and irregular ; and in still larger, but microscopical veins, these nuclei are seen to be mixed up with, or covered in by, more or less areolar tissue, which at first is destitute of nuclear fibre, but contains conspicuous nuclei, oval and irregular in shape and longitudinal in direction. In the accompanying drawing (fig. 860. a) is a small vein from the mesentery of a rabbit, measuring about of an inch in diameter, and treated with acetic acid. On the lining membrane are seen a few elongated nuclei and longitudinal striations, constituting a sort of incipient striated membrane. External to this is a thin layer of muscular tissue, and without that, again, is cellular tissue, which constitutes the main bulk of the vascular wall. The muscular coat is less distinct than in arteries of the same size and thinner in pro portion to the areolar coat.