A lecture on the human body / by John A. Parsons.

  • Parsons, John A., active 19th century.
Date:
1854
Catalogue details

Licence: Public Domain Mark

Credit: A lecture on the human body / by John A. Parsons. Source: Wellcome Collection.

Provider: This material has been provided by the National Library of Medicine (U.S.), through the Medical Heritage Library. The original may be consulted at the National Library of Medicine (U.S.)

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    one leading to the right lung and the other to the left. These branches divide and subdivide into almost countless tubes, until they are brought into contact with all the air-cells of both lungs. Having done this they unite into larger tubes called pulmonary veins, and these again into larger, until forming four great veins, two from each lung, they communicate with the left auricle. This cavity opens into the left ventricle. From this chamber arises the main artery of the body, called the aorta. This divides and subdivides into smaller and still smaller arteries, which are distrib- uted throughout the body. From these extend the capillary vessels, which are many times thinner than the finest hairs, and form a network through the whole structure of the body, impermeating every organ, every muscle, every bone. They even penetrate the nails, and the whole length of every hair. These capillaries com- municate with the veins which run in an opposite direc- tion to the arteries. These veins unite into larger ones, and these into still larger, until they form two great veins, one of which opens into the right auricle from above, and the other into the same cavity from beneath. Such is the structure of the heart, the arteries, the capil- laries, and the veins. What now causes the blood to circulate through these vessels % It is caused in part by the alternate contraction and relaxation of the four cavities of the heart. The open- ings that lead into these cavities, and out of them, are guarded by valves that permit the blood to pass through them on its way from the veins to the arteries, but pre- vent its return. The auricles contract when the ven-