368 



BLOOD-VESSELS. 



wherever a valve is placed (as in fig. 426, c). From the above description, it is 

 plain that the valves are so directed as to offer no obstacle to the blood in its 

 onward flow, but that, when from pressure or any other cause it is driven back- 



Fig. 426. DIAGRAM SHOWING VALVES OF VEINS. (Sharpey.) 



A. Part of a vein laid open and spread out, with two pairs 

 of valves. B. Longitudinal section of a vein, showing the 

 apposition of the edges of the valves in their closed state. C. 

 Portion of a distended vein, exhibiting a swelling in the situa- 

 tion of a pair of valves. 



wards, the refluent blood, getting between the 

 dilated wall of the vein and the flaps of the valve, 

 will press them inwards until their edges meet in 

 the middle of the channel and close it up. 



The epithelium cells differ in shape and arrange- 

 ment upon the two surfaces of the valves. On 

 the side which faces inwards, and past which the 



current of blood flows, the cells are elongated in the direction of the current, 

 whereas upon the opposite side which, when the valves are thrown back, faces the 

 wall of the vein, the cells are elongated transversely. The main substance of the 

 valve is formed by bundles of connective tissue, which have for the most part a 

 transverse arrangement, and between which a few elastic fibres are seen. The 

 tissue is covered on each surface by a prolongation of the inner coat of the vein, 

 the covering being much thicker on the inner than on the outer surface. The valve 

 is thinner close to its attachment than elsewhere. At its base a few transverse 

 muscular fibres are sometimes seen, prolonged into it from the middle coat. 



The valvular folds are usually placed in pairs as above described : in the veins of 

 the horse and other large quadrupeds three are sometimes found ranged round the 

 inside of the vessel ; but this rarely occurs in the human body. On the other hand 

 the folds are placed singly in some of the smaller veins, and in large veins single 

 valvular folds are not unfrequently placed over the openings of smaller entering 

 branches ; also in the right auricular sinus of the heart there is a single crescentic 

 fold at the orifice of the vena cava inferior, and another more completely covering 

 the opening of the principal coronary vein. 



Many veins are destitute of valves. Those which measure less than -^th of an 

 inch (about 2 millimeters) in diameter rarely, if ever, have them. In man, valves 

 are wanting in the superior and inferior venae cavse, in the trunk and branches of 

 the portal vein (except its gastric tributaries, Koeppe), in the hepatic, renal and 

 uterine veins ; also in the spermatic (ovarian) veins of the female. In the male, 

 these last-mentioned veins have valves in their course, and in each sex a little valve 

 is occasionally found in the renal vein, placed over the entrance of the spermatic 

 or ovarian. The pulmonary veins, those within the cranium and vertebral canal, 

 and those of the cancellated texture of bone, as well as the trunk and branches of 

 the umbilical vein, are rlso without valves. In the azygos and intercostal veins 

 valves are not generally found, and when present are few in number. On the other 

 hand, they are numerous in the veins of the limbs (and especially of the lower limbs), 

 which are much exposed to pressure in the muscular movements or from other causes, 

 and have often to support the blood against the direction of gravity. No valves are 

 met with in the veins of reptiles and fishes, and not many in those of birds. 



SMALLER ARTERIES AND VEINS AND CAPILLARIES. 



That the blood passes from the extreme arteries into the veins was a necessary 

 part of the doctrine of the circulation, as demonstrated by Harvey, in 1628 ; but 



