Such veins have, however, a strong reseni b!ance to small arteries. Henle states that the two systems of vessels are not to be dis tingiiished in those measuring up to of an inch ; and he appears to consider thinness of wall as the main characteristic in larger micro scopical veins. He instances one vessel, which he considers as venous, measuring A of an inch in entire diameter, having an annu!ar tunic and a cellular of an inch. It has appeared to me that the differences of these vessels are principally these, — micro-, scopical veins have thinner walls ; the taus cubic tunic is less distinct, thinner than the areolar, with which it is more or less mixed up, and the latter is more abundant and more developed than in small arteries.
In passing to vessels of larger size, the fibrous tissue is seen to have a more conspi cuous development. The longitudinal stri ations and fibres on the lining membrane form a perfect striated membrane, and, in still larger vessels, increase so as to constitute a compact coat of fibres, of more or less density — the internal tunic of longitudinal fibres. In the middle coat, the muscular element becomes involved and mixed up with areolar tissue, some fibres of which run parallel, and some at right angles to the muscular nuclei. The external tunic of areolar tissue becomes in creasingly developed, and yellow fibrous tissue conspicuous and abundant.* In fig. 860. A, is represented part of the renal vein of a rabbit. The vessel is not cut open or seen in section, but is simply laid upon glass, treated with acetic acid, and a small portion of the margin submitted to mi croscopical scrutiny. The internal membrane is densely striated, and the other coats de veloped in proportion.
There arc certain regions in which the veins undergo striking modifications in structure, which require description ; and the remarkable organs—valves—placed in their interior are yet to be considered.
These points will be examined as follows :— I. Veins, at their junction with the heart. IL Cam, passing through the diaphragm and pericardium.
III. Cerebral sinuses.
IV. Umbilical vein.
V. Venous valves.
I. At the junction of the veins wills the heart there is a gre it alteration in their composition, and this as it respects both cave, and the pul monary veins : they assume a muscular cha racter, and become thicker and red, which arises from a prolongation of the muscle of the auricle into the fibrous structure of the vein.
This anatomical fact, as far as the caviar are concerned, was described as early as 1664, by Borelli ; subsequently (1700), by Bidloo, who gave figures of it ; and afterwards (1779), by Gorter, who imagined that the muscle was continued into the smaller vessels, and that it there assumed a paler colour.
This muscular layer (though its existence is denied by Cruveilhier) may be followed upon the superior vena cava as far as the cla vicle, upon the inferior as far as the diaphragm, and upon the pulmonary veins as far as the division of the trunks into branches. (Ilenle.) It exists on the superior cava in greater amount than on the inferior, and it is there best submitted to examination. When viewed with the naked eye, the vena cava superior, at its junction with the heart, is seen to be red and muscular, and thicker than elsewhere. In following this away from the heart, the muscular character gradually diminishes, and the ordinary fibrous tissue assumes the pre ponderance till the former is altogether lost. At ten lines from the junction of.the vessel with the heart the mass was removed from which the accompanying drawing was taken.
The cavw of some birds are sufficiently thin and transparent to be viewed with transmitted light when entire, and when slit up and held to the light a beautiful arrangement of these fibres is seen : they do not form one flat, even, circular, covering, but are arranged in numerous, successive, interrupted, rings, very fine and red, gradually diminishing as they are seen further from the heart. I have observed this in the red-throated diver (Colynzbus sep tentrionalis), moor-hen (Gallinula ehloropus), and various other birds. According to Cuvier, in the ostrich, muscular fibres extend the whole length of the posterior cava, and dis appear suddenly opposite the kidneys.
When a little mass of this structure is ex amined with the microscope, it is found to consist of muscle of the same character as that of the heart, — the fibres being small, striped (though, perhaps, rather less distinctly so than that of the auricle or ventricle), and, as far as I have seen, destitute of sareolemrna. There is a good deal of wavy fibrous tissue, inter mixed with the muscle ; in this respect, con trasting remarkably with that of the heart : this exists in less amount in the immediate vicinity of that organ.