While Ccesalpinus had occasional faint glim merings of the physiology of the veins, and Fabricius, a remarkable knowledge of their anatomy, it was still left to Harvey to indi cate their function and relation to the general circulation.* Veins are the necessary companions of arteries, and are consequently found in all animals possessing the latter vessels.
In the following remarks I shall, for the purpose of examining the subject more com pletely, divide the consideration of vein into these heads : I. Structure.
II. Physical and vital properties.
Ill. Origin, course, anastomoses, plexuses, IV. Function.
V. Development.
My observations will be principally con fined to the consideration of these various heads as applied to the veins of mammalia, especially the human subject. The general anatomy of this subject in invertebrate classes has not, as yet, been sufficiently examined, and does not appear to furnish many points for generalisation beyond those supplied in the mere anatomical description given of the venous system in the several invertebrate classes, in the articles specially applied to them.
I. STI•ICTURE.—There is scarcely any sub ject in structural anatomy, which has given rise to so many varied and discrepant opinions, and so much contradictory description as the structure of veins ; the different writers upon this point having been numerous, and with scarcely any exception, each giving an ac count in sonic, and often important, respects, contrary to those who have preceded him. This circumstance may, to a certain extent, be explained by the various observers having ex amined different veins, in different regions, and in different animals .; and it must be remem bered that all microscopical observations necessarily differ from those which are made with the scalpel and the naked eye, and are incompatible unless conjoined, compared and interpreted by the same individual ; for, as we shall presently find, structures which seem reduced by dissection to their simplest ele ments, are found, when submitted to the mi croscope, to be compound ; this is especially the case as it regards the internal tunic of veins, as displayed by coarse anatomy and the microscope respectively. All the discrepancies, however, which have occurred cannot be settled upon these grounds, and it must be granted that in some instances the writers upon this subject have rather drawn upon their imagination than depended entirely upon anatomical demonstration. These conflicting
statements, however, serve to keep bore us the fact that there is considerable difference of structure in veins in certain regions.
The early anatomists who devoted their at tention to the investigation of the intimate structure of organs, applied themselves to the study of the minute texture of veins, and it was indeed one point in their structure which was greatly instrumental in leading Harvey in his discoveries of the circulation.
Constantin= first described the structure of veins as consisting of a " tunica villosa." Ve salius speaks of the membranous character of veins, and of their being composed of three sets of fibres, — a longitudinal, a circular, and an oblique. Fallopius and Bartholini deny the fibrous nature of the coats of veins, and Dieniebroeck described veins as consisting of one membranous tunic, and believes the state ment of the existence of three tunics to be mere imagination. Willis, Icicolai, and Blau card describe veins as being composed of four coats or tunics. Haller denies the existence of transverse muscular fibres in the coats of veins, which had been repeatedly mentioned by other anatomists. Lieutaud states that the veins are identical in structure with the ar teries, but simply attenuated. Prochaska does not admit the existence of a fibrous tunic in veins, whilst Scemmerring says it is to be found only in the larger ones. Meckel, Autenrith, and Bichat deny altogether the circular or transverse coat of veins, and Senac says that their tunics are composed simply of longitu- . dinal fibres.
To these might be added a long list of di verse descriptions, which are however of more literary curiosity than anatomical value.
Veins, in the human subject and mammalia, and I believe in vertebrata generally, are membranous cylinders, consisting of various fibrous coats, lined internally by an epithelium. The walls of veins, which are sufficiently thick to admit of coarse dissection, are with care divisible by the scalpel into three layers, an internal, middle, and external coat : but, when submitted to microscopical scrutiny. a still further analysis is made; the internal layer is found to have a compound character, and the direction of the elementary fibres of the several tunics is also shown ; the internal and external coats are seen to be longitudinal, while the middle is compound, partly circular and partly longitudinal.