NATURAL H1STOR1 29 



" At thirty man suspects himself a foo., 

 Knows it at forty, and amends bis plun." YOUNO. 



75. IVliy have veins a blue or black appearance, although blood is 

 of a red colour ? 



Because the oxygen which the blood originally contains is ab- 

 sorbed as it passes through its course of circulation, and the 

 blood is discoloured. But when it is again sent through the lungs, 

 it receives a fresh supply of oxygen and regains its colour. 



76. Wliy are the veins sometimes seen to swell ? 



As long as the veins that proceed from the organs are free, the 

 blood that arrives in them from the arteries runs on and does not 

 accumulate in them ; but if the veins are compressed, or cannot 

 empty themselves of the blood always arriving by the arteries, and 

 finding no place in the veins, accumulates on the tissue of the 

 organ, distends the blood-vessels, and augments, more or less, its 

 volume, particularly if its physical properties can undergo these 

 changes. The swelling of the brain, by the difficulty of the circula- 

 tion happens every time that the blood has more difficulty in passing 

 through the lungs ; and as that generally takes place in expiration, 

 the brain swells in this instance so much more in proportion as 

 the expiration is more complete and of larger continuation. 



77. Why is a ligature tied round the arm when it is bled ? 

 Because the veins have valves placed in them at certain distances, 



which prevent the blood from flowing backward from the heatf 

 towards the extremities ; when the ligature is tied the blood is 

 constantly arriving from below, because the pressure is not great 

 enough to obstruct the arteries, but it cannot get up past the 

 bandage ; the veins are therefore distended and become prominent, 

 so as easily to be seen and punctured ; and then as the blood cannot 

 get down the arm again on account of the valves, it is necessitated 

 to flow out at the orifice. 



78. JVTiy does moderate cold act as a stimulant to the system? 



Because cold repels the blood from the surface, braces and con- 

 tracts the integuments, and lessens the transpiration. The thinner 

 and superfluous quantity of the circulating fluid is no longer 

 thrown from the surface, but is drained off in the fluid state, and 



