OF THE VEINS. 297 



parietes and the great number of anastomoses. The commu- 

 nication of the veins present all the varieties already indi- 

 cated [356,] and moreover, the union of the very large trunks, 

 as that of the venae cavae by the vena azygos; the union of su- 

 perficial and deep seated veins, as that of the cranial and spinal 

 veins with the epicranial, temporal, cervical, &e., that of the 

 internal and external jugular veins, and that of deep seated 

 veins, with the subcutaneous veins of the limbs. 



Generally, the veins have a less flexuous, straighter, and 

 therefore, shorter course than the arteries. 



The variations of the veins have been a little exaggerated, 

 and those of the arteries have been somewhat concealed. The 

 large venous trunks especially are less variable than they 

 have been announced; but the branches and twigs are very 

 much so. 



446. The interior of the veins present a great number of 

 valves* or folds of the inner membrane, which constitute a 

 great difference between them and the arteries. The valves 

 are very well seen by examining under water a vein split open 

 lengthwise. 



Each valve consists in a fold of the inner membrane. This 

 fold has a convex edge, adhering to the parietes of the vein 

 on the side towards its radicles, and a concave and free mar- 

 gin, turned towards the heart. These two edges are some- 

 what thicker than the rest of the fold; one of the surfaces 

 is turned towards the cavity of the vessel, and corresponds 

 to the circulating blood, the other corresponds to the parietes 

 of the vein, somewhat dilated at this point. When the valve 

 shuts, the surface, which corresponds to the radicles, becomes 

 convex, the other becomes concave, and the vein slightly en- 

 larges; the valves are so much the more broad as the vein is 

 the more voluminous, and so much the more elongated as it 

 is smaller. It is to this difference especially that the varieties 

 of form described by Perrault and several others is to be re- 

 ferred. 



* H. Fabricio, de venarum ostiolis, in op. omn.J. G. Schmiedt and Mei- 

 bomius, de valvulls seu membranulis vasorum earumque struct, et usu, Helmst 

 1682. Perrault Essais de physique, torn. iii. 



