_ ANGEIOLOGY. 77 
The blood, when returned from the capillaries through the veins, is no 
longer fit for the purposes of life, and must accordingly be purified, or freed 
from the dead matters with which it is loaded. This purification is mainly 
effected through the instrumentality of the lungs, in which the venous blood 
comes in contact with the atmosphere and constitutes respiration. In the 
lungs, the oxygen of the air is absorbed by the blood, and uniting with the 
-superabundant carbon, forms carbonic acid, which, as a gas, may be exhaled. 
The accession of oxygen converts the dark venous blood into the bright red 
arterial, which again returning to the heart, is impelled into the arteries as 
before. This union of the carbon of the blood with oxygen absorbed from 
the air, takes place not only in the lungs but in the bloodvessels. The 
circulation of the blood through the arteries, capillaries, and veins, is known 
as the greater or systematic circulation, that through the lungs being the 
lesser or pulmonic 
1. SPECIAL ANATOMY OF THE HEART. 
The central organ of the vascular system, the heart, is a hollow, irregu- 
larly conical body, slightly flattened posteriorly, and so situated in the tho- 
racic cavity between the two lungs, as that its base from which the blood- 
vessels arise 1s superior, and the apex directed downwards and to the left. 
The heart does not lie loose in the thorax, but is inclosed in a membranous 
bag called the pericardium. The external or fibrous layer is closely united 
to the pleura or liming membrane of the thorax, and to the mediastinum, 
a nearly vertical partition formed by the juxtaposition of the pleure of 
opposite sides. Although, on the whole, of similar shape to the heart, it is 
yet inverted, the apex being superior and the base inferior. Below it is 
firmly united to the tendon of the diaphragm. It consists essentially of 
two layers, an external or fibrous, and an internal or serous. The serous 
layer is reflected over the heart, and secretes a thin yellowish fluid, the 
liquor pericardu, which lubricates the heart, and permits it to play freely 
within its pericardium. The amount of this liquor when in health seldom 
exceeds a teaspoonful. 
The heart is a strong muscular bag, divided into two compartments, two 
auricles and two ventricles. On looking at it from before, there will be 
seen a longitudinal furrow, sulcus longitudinalis, which divides it into right 
and left portions; this furrow corresponds to the internal partition dividing 
the right auricle and ventricle from the left auricle and ventricle. A second 
furrow, sulcus transversalis or coronalis, intersects the first at right angles, 
and marks the partition between the right and left auricle and the right and 
left ventricle. The two auricles form the base of the heart, the ventricles 
constituting its body; and the anterior end of the left ventricle, by being 
extended somewhat beyond the right, forms the apex. ‘The auricles are in 
immediate connexion with the great venous trunks; the right with the two 
venz cave, the left with the pulmonary veins, all of which conduct blood 
to the heart. 
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