CAPILLARIES. 



175 



crescentic fold at the orifice of the vena cava inferior, and another more com- 

 pletely covering- the opening of the principal coronary vein. 



Many veins are destitute of valves. Those which measiu'e less than a line m 

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

 the superior and inferior venaj cava3, in the trunk and branches of the jioital 

 vein, in the hepatic, renal, and uterine veins ; also in the t-pcrmatic (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. The pulmonary veins, those within 

 the cranium and vertebral canal, and those of the cancellated textui-e 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 nmnber, 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 support the 

 blood against the direction of gravity. No valves are met with in the vems of 

 reptiles and fishes, and not many in those of birds. 



Fis. llo. 



SMALLER ABTERIES AND VEINS AND CAPILLARIES. 



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 micro- 

 scope 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 

 moderate power (as in fig. 115), 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 batra- 

 chians, and in the tail and gills of their 

 larvas : also in the tail of small fishes ; 

 in the mesentery of mice or other small 

 quadrupeds ; and generally, in short, in 

 the transparent vascular 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 mem- 

 branous parts, such as those above men- 

 tioned, but also in more thick and opaque 

 tissues, which can be subsequently rendered 

 transparent. 



The capillary vessels of a part are most 

 commonly arranged in a network, the 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 



Fig. 115. — Capillary Blood- 

 vessels IN THE Web of a 

 Frog's Foot as seen with the 

 Microscope (after Allen Thom- 

 son). 



The arrows indicate the course 

 of the blood. 



