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 is 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 i\iQ pericardium. The external or fibrous layer is closely united 

 to the pleura or lining membrane of the thorax, and to the mediastinum, 

 a nearly vertical partition formed by the juxtaposition of the pleurae 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 pericardii, 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 farrow 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 

 venae cava?, the left with the pulmonary veins, all of which conduct blood 

 to the heart. 



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