CAPILLARY VESSELS. clxxv 



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

 backwards, 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 chumel and close it up. 



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

 the horse and other large quadrupeds three are often found ranged round the inside of 

 the vessel ; but this rarely occurs in the human body. On the other hand, the valves 

 are placed singly in some of the smaller veins, and in large veins single valves 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 a line in 

 diameter rarely, if ever, have them. In man, valves are wanting in the trunks of the 

 superior and inferior venae cavae, in the trunk and branches of the portal vein, in the 

 hepatic, renal, and uterine veins ; also in the spermatic 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. 

 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 without valves. Valves are not generally found, and when present are few in 

 number, in the azygos and intercostal veins. 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 sup- 

 port 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. 



CAPILLARY VESSELS. 



That the blood passes from the arteries into the veins was of course a 

 necessary part of the doctrine of the circulation, as demonstrated by Harvey; 

 but the mode in which the passage takes place was not ascertained until 

 some time after the date of his great discovery. The discovery of the 

 capillary vessels, and of the course of the blood through them, was destined 

 to be one of the first fruits of the use of the microscope in anatomy and 

 physiology, and was reserved for Malpighi (in 1661). 



When the web of a frog's foot is viewed through a microscope of mode- 

 rate power (as in fig. xciv.), the blood is seen passing rapidly along the 

 small arteries, and thence more slowly through a network of finer 

 channels, by which it is conducted into the veins. These small vessels, 

 interposed between the finest branches of the arteries and the commencing 

 veins, are the capillary vessels. They may be seen also in the lungs or 

 mesentery of the frog and other batrachians, and in the tail and gills of 

 their larvae : also in the tail of small fishes ; in the mesentery of mice or 

 other small quadrupeds ; and generally, in short, in the transparent vas- 

 cular parts of animals which can readily be brought under the microscope. 

 These vessels can also be demonstrated by means of fine injections of 

 coloured material, not only in membranous parts, such as those above- 

 mentioned, but also in more thick and opaque tissues, which can be rendered 

 transparent by drying. 



The capillary vessels of a part are most commonly arranged in a network, 

 tho branches of which are of tolerably uniform size, though not all strictly 

 equal ; and thus they do not divide into smaller branches like the arteries, 

 or unite into larger ones like the veins ; but the diameter of the tubes, as 

 well as the shape and size of the reticular meshes which they form, dnTers in 



