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Around the bronchial tubes and their ramifications there are also arteries and veins called the bronchial, arising from the vena azygos or vena cava, and from the aorta. These arteries and veins are distinct from the pulmonary arteries and veins. From this it is evident that the blood flows into the lungs by two ways, and flows out from them by two ways.
But what earthly use has a man for valves in the intercostal veins which carry blood almost horizontally backward to the azygos veins? When recumbent, these valves are an actual obstacle to the free flow of the blood. The inferior thyroid veins which drop their blood into the innominate are obstructed by valves at their junction.
It entered the back near the left clavicle, beneath the scapula, close to the vertebrae between the intercostal spaces of the fifth and sixth ribs; grazing the pericardium it traversed the mediastinum, barely touching the oesophagus, and vena azygos, but completely severing the thoracic duct, and lodging in the xiphoid portion of the sternum.
Supplementary pulmonary lobes are occasionally seen in man and are taken by some authorities to be examples of retrogressive anomalies tending to prove that the derivation of the human race is from the quadrupeds which show analogous pulmonary malformation. Eckley reports an instance of supernumerary lobe of the right lung in close connection with the vena azygos major.
Clevenger points out that the intercostal veins, which carry blood almost horizontally backward to the azygos veins and which would run vertically upward in quadrupeds, possess valves. These are not only useless to man, but when he lies upon his back they are an actual hindrance to the free flow of the blood.
Of the few exceptions to this rule, he considers the valves of the jugular veins as in process of becoming obsolete, and the rudimentary azygos valves as a recent development.
An apparent anomaly exists in the absence of valves from parts where they are most needed, as in the venæ cavæ, spinal, iliac, hæmorrhoidal, and portal. The azygos veins have imperfect valves.
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