This middle coat is very compact, especi ally near the inner surface, and it gradually becomes less so in proceeding outwards.
It is difficult to tell what is the course and direction of the nidus of white fibrous tissue in which the yellow element is embedded, as in the middle coat it forms a dense granular compact mass*, when seen in situ, and its true nature is only displayed by the action of acetic acid. When, however, portions of the tissue are picked abroad with needles, though its direction is lost, its characters are ob vious.
As it regards the muscularity of this coat, there can exist no doubt. The recent ob servations of Kolliker on the low form of muscle, which he designates " muscular," or "contractile fibre-cells," have placed this pre viously obscure subject in an intelligible and satisfactory light, and have done much to ex plode the idea of non-muscular contraction, by exhibiting the wide diffusion of this hitherto unrecognised tissue.
KBIliker describes two forms of fibre-cells as existing in blood-vessels,— one, consisting of short, round, spindle-shaped, or rectangular plates, like epithelium ; the other, of long plates of irregular, rectangular, spindle, or club, shape. The substance of these cells is soft, light yellow, and homogeneous, and each con tains a peculiar, characteristic nucleus, whose shape is constant, being that of a club or staff. These fibre-cells are placed transversely as it regards the vessel, and constitute a thin coat, immediately external to the lining membrane, intermixed with cellular tissue. These mus cular elements are clearly seen (as exhibited by their nuclei) by the action of acetic acid on small vessels. (Kg. 860.) He further states that the great develop ment of the uterine veins during pregnancy is principally from an increased size of the fibre cells existing in the middle coat, but partly also from the inner and outer coat acquiring a considerable quantity of smooth muscular fibre. According to the same authority this mus cular element is not found in the veins of the uterine portion of the placenta, the cerebral veins, the cerebral sinuses, Breschet's veins of the bones, and the venous cells of the corpora cavernosa.
External coat of longitudinalfibres. —The ex ternal coat consists of a mass of areolar tissue, adhering together with more or less compact ness, and running in a longitudinal direction.
This tunic occupies full two-thirds of the entire thickness of the whole wall of a vein. Its internal boundary is irregular at the union with the equally irregular outer limit of the middle coat, and its external boundary is the loose cellular tissue into which it degenerates, and which constitutes the "sheath" of the vessel. This coat, like the preceding, gradu ally diminishes in density and compactness as it is examined further and further from its inner limit, though its density is everywhere inferior to that of the middle ; indeed, it may be said that the tissue of which the vessel is composed becomes looser and looser in pro ceeding outwards from the inner boundary of the middle coat,— this quality passing gradu ally from one coat to the other.
The textural arrangement of this tunic must be examined, like the preceding, from longitudinal and transverse sections. In the former it is seen that bundles of rods of yel low elastic tissue are disposed in alternate laminm with white fibre, or rather, that the former are embedded in a mass of the latter, and that they are a continuation of the longi tudinal fibres of the middle coat, which gradu ally become arranged more and more in strata, and at increasing distances. (Fig. 856. c.) The characters of the white fibrous tissue between the bundles of yellow, become more conspicuous as the laminm are wider apart.
The stratified arrangement of the fibres, and their correspondence with the longitudinal rods of the middle coat are very well seen in the specimen treated with acetic (fig. 857.).
When this tunic is viewed in transverse section, the relative thickness and proportion of the lamellm are better seen than in a longi tudinal cut : the yellow element is seen to occupy but a small relative thickness in com parison with the white.