GENERAL CONSIDERATIONS. 683 



parieties ; and between the vena portse and the deep epigastric and vesical 

 veins. 



Form. — "With regard to form, the veins represent cyhndrical tubes, sHghtly 

 knotted on those parts of their course which correspond to their valves, but they 

 are rarely distended as the arteries are ; the only exceptions are found in the 

 venous polyhedral spaces or sinuses in the dura mater. 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, so that all the 

 branches collected at last into an imaginary single canal would form a hollow 

 cone, the apex of which would correspond to the auricles. 



Number and Capacity. — In comparing the two orders of vessels with 

 reference to their number and capacity, we discover notable differences. 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 repre- 

 sentatives 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 system, and that we may 

 consider the relation of two to one as being the approximate 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 dis- 

 position favours the flow of the blood in the veins, at the commencement of 

 which 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 introduction of 

 air into the circulatory system — especially during inspiration — when a vein is 

 opened. 



Internal Conformation. — The interior of the veins is remarkable for the 

 presence of valvular folds, the arrangement of which resembles, in principle, that 

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

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

 towards the heart when the valves are tense ; and a convex surface which, on 

 the contrary, looks towards the roots of the veins. 



These valves are most frequently single, but they are sometimes joined in 

 twos or threes ; according to some authorities, there are even found four or five 

 together, arranged in a circular manner. They are distinguished as parietal and 

 ostial valves, the latter being placed at the entrance of one vein into another. 

 All the veins are not provided with them, however, and where they exist they 

 may be more or less numerous. They are absent in the pulmonary system, and 

 in the trunks of the venaB cava ; absent or quite rudimentary throughout the 

 extent of the vena portae ; rare and sUghtly developed in the vena azygos, the 

 veins of the testicle, uterus, and ovary ; and very numerous, very large, and 

 very complete in the veins of the limbs. 



The function of these valves is to favour the onward course of the blood, 



