GENERAL CONSIDERATIONS. 597 



form a series of convergent ramifications which repeat, in a general manner, 

 but in an inverse sense, the arterial ramifications whose course they for 

 the most part follow. A certain number, nevertheless, are placed at some 

 distance from the arterial trunks, beneath the external tegumentary 

 membrane, where they are disposed in a vast network which constitutes 

 the superficial veins of the body. Apart from this peculiarity, we have 

 nothing more to say with regard to the situation, direction, relations, and 

 anastomoses of the veins than has been already made known in studying 

 the arteries. It is only to be remarked, that the anastomoses of the venous 

 system are more numerous, larger, and more complicated, than those 

 of the arterial system ; that they also communicate with more voluminous 

 trunks ; and that they very often join the deep to the superficial veins. 

 At certain points (external genital organs, bladder, rectum), the anastomoses 

 are so numerous as to constitute veritable venous plexuses. These are more 

 especially met with in regions where the circulation is exposed to be more 

 or less hindered, either from the displacement of organs or variations in 

 their volume. 



With regard to form, we also find a close analogy between the veins 

 and arteries. The majority of the first, at least, represent as do the second 

 cylindrical tubes, slightly knotted, it is true, on those parts of their track 

 which correspond to their valves ; the only exceptions are found in the 

 venous dilatations of the dura mater polyhedral spaces which are designated 

 sinuses. On the other hand, veins offer the same collective form as the 

 arteries ; the general volume of the venous ramifications being as much more 

 developed as they are distant from the heart, all the branches collected at 

 last into an imaginary single canal would form a hollow cone \vhose apex 

 would correspond to the auricles. 



It is only in comparing the two orders of vessels with reference to their 

 number and capacity, that we can discover any sensible difference. The 

 veins are more numerous than the arteries, as a great number of the latter 

 are accompanied by two of the former, and the subcutaneous veins have no 

 representatives in the arterial system. All the veins being, besides, much 

 more voluminous than the corresponding arteries, it follows that the total 

 capacity of the venous system much surpasses that of the arterial tree, and 

 that we may boldly consider the relation of two to one as being the approxi- 

 mative expression of this difference. 



When the veins are compared with the arteries, it is remarked that the 

 relations between the length of the trunks and branches are reversed. In 

 the arteries, the trunks are large and the branches short ; in the veins, on 

 the contrary, the branches are relatively much longer than the trunks. 

 This disposition favours the flow of the blood in the veins, at whose 

 commencement we do not find, as in the arteries, a propelling organ. 



Another arrangement to answer the same end, in opposing the influence 

 that atmospheric pressure might have on the veins, is the attachment of 

 these vessels to the walls of the cavities they pass through ; this is observed, 

 for instance, at the entrance to the thorax. 



This disposition, so favourable to the circulation of the blood, becomes 

 a constant source of danger to the surgeon, by its permitting the intro- 

 duction of air into the circulatory system when a vein is opened. 



INTERNAL CONFORMATION. The interior of the veins is remarkable for 



the presence of valvular folds, whose disposition resembles, in principle, that 



of the sigmoid valves of the heart. These veins offer : an adherent border 



attached to the walls of the vessel ; a free, semilunar border ; a concave 



41 



