Quain's elements of anatomy . d those of the cancellated texture of bone, aswell as the trunk and branches of the umbilical vein, are also without the azygos and intercostal veins valves are not generally found, and whenpresent are few in number. On the other hand, they are numerous in the veinsof the limbs (and especially of the lower limbs), which are much exposed topressure in the muscular movements or from other causes, and have often tosupport the blood against the direction of gravity. No valves are met with inthe veins of reptiles and fishes, and not many in those of birds. SM
Quain's elements of anatomy . d those of the cancellated texture of bone, aswell as the trunk and branches of the umbilical vein, are also without the azygos and intercostal veins valves are not generally found, and whenpresent are few in number. On the other hand, they are numerous in the veinsof the limbs (and especially of the lower limbs), which are much exposed topressure in the muscular movements or from other causes, and have often tosupport the blood against the direction of gravity. No valves are met with inthe veins of reptiles and fishes, and not many in those of birds. SMALLER ARTEEIES AND VEINS AND CAPILLARIES. That the blood passes from the extreme arteries into the veins wasa necessary part of the doctrine of the circulation, as demonstrated byHarvey, in 1628; but the mode in which the passage takes -place wasnot ascertained until some time after the date of his great finding of the capillary vessels, and of the com^se of the blood SMALLER ARTEEIES, VEINS AND CAPILLAEIES. 193. through them, -was in fact one of the first fruits of the use of tlie micro-scope in anatomy and physiology, and was reserved for Malpighi (in 1661).When the web of a frogs foot is viewed through a microscope ofmoderate power (as in fig. 187), the blood is seen passing rapidly alongthe small arteries, and thence more slowlythrough a network of finer channels, by Fig. 187. which it is conducted into the small vessels interposed between thefinest branches of the arteries and the Fig. 187.—Capillary blood-vessels in the web OF A frogs foot, AS SEEN WITH THE MICROSCOPE (after Allen Thomson).The arrows indicate the course of the blood. commencing veins, are the capillary vessels. The course of the blood in them may be conveniently seen also in the lungs or mesentery of the frog, in the external gills and tail of tadpoles ; in the tail of small fishes ; in the mesentery of small quadrupeds ; and generally, in short, in the transparent vascular par
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