272 PHYSIOLOGY. 



tube of eel-skin, or any thing which is not elastic, and you 

 attempt to pump the water out of it, supposing it to be filled, 

 the sides will be brought together, and the tube closed, so 

 that the water cannot escape. This, however, will not hap- 

 pen, if, as in the veins, the fluid is forced in at the other end. 



43. The expansion of the chest in breathing, also aids in 

 circulating the blood in the veins. When the chest is dila- 

 ted, both air and blood rush into it. This may be seen by 

 watching the jugular veins in the neck, which empty them- 

 selves during inspiration. Dr. Barry placed one end of a 

 tube in the jugular vein, and the other in a coloured fluid. 

 During inspiration, the fluid was sucked from the vessel into 

 the vein ; during expiration, it remained stationary. It should 

 be remembered, that during one act of respiration, the heart 

 beats five or six times. 



44. One other cause remains to circulate the blood in the 

 veins, and that is, the action of the muscles. When the 

 muscles contract, they press upon the veins in contact with, 

 or near them, and so force the blood along their cavities. 

 This can be seen in bleeding from the arm ; if a person 

 grasps a stick, the blood flows much more freely than when 

 the muscles are relaxed. It is in this way, that exercise 

 proves so beneficial to health, by promoting the circulation 

 of the blood through the system ; and we account in this 

 way also, for the fact that sedentary habits so often lay the 

 foundation for incurable diseases. 



45. (The heart is not so dependent on the brain for its ac- 

 tion as many other organs. } ^lonsters, born without heads, 

 sometimes live for several days. Snakes have lived six 

 months without a head ; and any animal may live for some 

 time in the same condition, if the blood-vessels of the neck 

 are tied. If the breathing be kept up by artificial respiration, 

 life may be continued for a long time. I have myself kept 

 a child alive two hours, that had its neck broken, by keeping 

 up artificial breathing ; persons have been saved in the same 

 way, who had taken large doses of laudanum. 



