1368 



VEIN. 



norance, as to the bearing of the veins upon 

 the circulating system. Vesalius endeavoured 

 to establish the doctrine, that the vena cava 

 takes its origin from the heart, and not the 

 liver ; but in this he was opposed by Sylvius, 

 Columbus, Eustachius, and Fallopius. 



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. 



III. Origin, course, anastomoses, plexuses, 

 &c. 



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. STRUCTURE. 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 some, 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. AH the discrepancies, 

 however, which have occurred cannot be 

 settled upon these grounds, and it must be 



* For more elaborate details of literary history, 

 I would refer the reader to the article CIRCULATION, 

 and to the learned " Diatribe Anatomico-physio- 

 logica de structura et vita venarum," by Marx. 

 See BIBLIOGRAPHY. 



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 be'ore 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. 



Constantinus 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. Fa/lopius and Bartholini deny the 

 fibrous nature of the coats of veins, and 

 Diemebroeck described veins as consisting of 

 one membranous tunic, and believes the state- 

 ment of the existence of three tunics to be 

 mere imagination. Willis, Nicolai, and Blan- 

 card describe veins as being composed of four 

 coats or tunics. Hatter 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. 



The relation of coarse and microscopical ana- 

 tomy will be best seen by the following table : 



Coarse Dissection. Microscopical Examination. 



' Epithelium. 

 Internal coat - Fenestrated membrane. 



Longitudinal fibres. 



-.jr-i i, f Circular and longitudinal 



Middle coat ^ fibres intermixed. Muscle. 



T-, . , f Longitudinal fibres ; corn- 



External coat | pa fe t areo)ar ti 



