This section is from the book "Human Physiology For The Use Of Elementary Schools", by Charles Alfred Lee. Also available from Amazon: Human Physiology, for the Use of Elementary Schools.
40. The capillary vessels are the last part of the body that continues to act. After the breathing and the action of the heart have ceased, they still continue to act like innumerable little pumps, drawing the blood out of the arteries and substance of the organs, and forcing it into the veins. As nutrition and secretion are performed by that portion of the capillary system which acts independently of the heart and arteries, the continuance of action in this system accounts for the growth of the beard and the hair, which takes place after death. It is owing to the same reason, that the arteries are always found empty after death.
41. Physiologists are not agreed as to the cause of the motion of blood in the veins. The veins have thinner coats than the arteries and are destitute of elasticity. As they are wanting in elasticity, if they had no irritability, they could not act upon the blood contained in them, and accordingly could exert no active force in circulating the blood. But it is found by experiments that the veins are not mere passive tubes.; they possess a certain degree of contractile power, as is shown in the shrinking of the veins on the back of the hands in cold weather; besides, if a vein be punctured between two ligatures, the blood will spirt out. The veins then assist the circulation by a sli ht degree of contractile power.
42. Again : when the heart dilates, the blood is sucked up in the veins precisely as it is in a pump. This is denied by some, who say, that if the end of a syringe be placed in a 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 happen, 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 dilated, both air and blood rush into it. This may be seen by watching the jugular veins in the neck, which empty themselves 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 action as many other organs. Monsters, 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.
46. In fainting, the heart ceases to act. It may be owing to various causes, acting on the nervous system, or on the blood vessels. Mental emotion, loss of blood, or any thing that renders the blood vessels about the heart less full or tense than usual, will cause a person to faint. This state is soonest relieved by lying down, probably because the action of the heart is sufficient to force the blood along horizontal tubes, but not to raise it in a perpendicular position. Where blood has been lost in sufficient quantity to endanger life, it has been supplied from the veins of another person, by a process called transfusion.
When was the circulation of the blood discovered? and by whom ? Why not discovered before ? Why is the circulation called a vital function ? How is life maintained ? Have all animals a circulation ? Describe the structure of the heart. What is the pericardium ? In what animals is the heart double ? Describe the cavities of the heart-the valves. What is their use ? What are arteries ? How many coats have they ? Describe the venous system. Have the veins valves ? What is the capillary system ? Describe the circulation in reptiles-worms-fishes. What faets did Harvey bring forward to prove the circulation of the blood ? What is the motion of the blood chiefly owing to ? Are the ventricles thicker than the auricles ? Why ? What is said of the velocity of the blood,? Of the force of the heart ? Describe the composition of the blGod ? What proportion of the body does it constitute ? How long is the blood in performing a revolution ? What is the last part of the body that continues to act ? Why are the arteries found empty after death? What circulates the blood in the veins ? What aids in this process ? Does the heart depend on the brain for its action ? What facts connected with this subject ? What occurs in fainting ? What position is most favorable in recovering from fainting ?