There is yet another peculiarity in the structure of the great veins near the heart,— they have a partial investment of the serous layer of the pericardium ; this, however, exists only on the anterior surface ; and in the in ferior cava the amount of serous covering is very small.
II. When the came pass through the peri cardium and diaphragm they contract an inti mate connection with those structures whose fibrous tissue is more or less reflected upon them and adds to their coats. Upon the su perior cava are to be seen (external to the pericardium) white, glistening fibres, having a longitudinal course, and traceable from the pericardium directly upon the vein. The pulmonary veins have a similar covering.
In the human subject, the inferior cava passes through the pericardium and diaphragm at once, and there forms so intimate an ad hesion to the margin of the aperture that I have removed striped muscle (of the dia phragm) from the fibrous tissue of the vessel, it having taken its origin from the outer tunic of the vein.
In many of the lower animals (as in the sheep, cat, and rabbit,) there is a considerable interval between the diaphragm and pericar dium, in which the vena cava inferior is in vested with a covering of fibrous tissue con tributed from both these sources. It is white, glistening, and strictly longitudinal, and is nished in amount by the pericardium.
III. Cerebral cerebral sinuses hold the place of large veins to the brain, but are materially different from large veins in structure. They consist of excavations or tubes in the substance of the dura mater, — the dura mater is, as it were, split into two layers, and the interval is the cavity of the sinus. Their form, which is very irregular— triangular, quadrilateral, &c.—differs in dif ferent situations.
The internal or lining membrane of the sinuses is stated by all anatomists to be iden tical with that of the veins ; but its exact amount and nature I have not myself ascer tained. The continuity of the lining mem brane with that of the veins is seen where the sinuses join the veins ; thus, at the ca vernous sinus, where the ophthalmic vein and circular sinus join it, the lining membrane adheres to the inner wall of the sinus, but is separated from the outer by the carotid artery and certain nerves. The other element of the walls of these peculiar veins is the white fibrous tissue of the dura mater, arranged without any regularity. The inner surface of these vessels is smooth, but rendered ex tremely irregular by the occurrence of nu merous fibrous processes on bands projecting into their cavity. In the superior longitudinal sinus, the cavernous, and middle basilar, this is most conspicuous. In the first mentioned
of these, the fibrous bands have been called "cords' Willisiance" (after Willis, their first describer). They are slips of white fibrous tissue passing from the sides of the sinus, es pecially near the angles, attached at both extremities, and either free or attached along one side : these give the appearance as if the inner surface were divided into cells ; into some of these the probe enters and passes on into the veins on the surface of the brain; others are blind, or lead to lesser sinuses, which not un frequently run parallel, for some length, to the great sinus ; or the probe passes from one of these cells to another. In the cavernous sinus these fibrous hands are so numerous that they look like a mass of areolar tissue, whose areolm are distended with blood. The cord Willisianm are traversed by minute arteries. (Sir C. 13-11.) IV. The umbilical vein has a structure quite peculiar to itself. The vessel may he best examined by splitting it up with a director and pinning it down on cork, with its inner surface upwards. It is then seen to be smooth, valveless, and whiter than other veins.
From the inner surface I have obtained epithelium : the cells were flat, irregular in outline, granular and fatty.
On making an incision, the membrane of which it is composed retracts, so as to leave a gap, and displays the dense fibrous mass which forms the basis of the cord. It tears off in irregular shreds, not showing a tendency to tear in any particular direction. With care, considerable portions of the membrane may be stripped off. This structure is soft, elastic, and semi-pellucid. It appears structureless to the naked eye, and looks almost like a thin film of fecula jelly (arrow-root made with water), or rather, perhaps, like the flaccid dull cornea of an animal dead some days. When separated, it coils up, and, where the surfaces have become adherent, it is difficult to unfold it again : it now looks and feels like thick mucus—it is semitransparent and adhesive. When seen under the microscope, it is found to be indis tinctly fibrous ; some masses appear as a dense web of flat fibres, the fibres being strictly on the same plane, with their sides adherent at some points, and leaving intervals at others ; in some places the interspaces are only small specks on the surface of what appears to be in other respects, an almost homogeneous sheet ; there are also some indistinct longi tudinal striations, connecting these minute interspaces and obscurely indicating the out line of fibres. The element of which this coat is composed is singularly pellucid under the microscope, scarcely refracting excepting at its edges.