STRUCTURE OF THE HEART. 321 



The opening of the aorta, anterior to that of the auricle, is next the 

 septum of the ventricles. Its aperture is round, and rather smaller than 

 that of the pulmonary artery, and measures about three-quarters of an incli 

 in diameter. It is situate opposite the inner end of the third left intercos- 

 tal space. 



In its interior are three semilunar or sic/moid valves, which are larger 

 and stronger than the corresponding parts in the pulmonary artery, but 

 have a like structure and attachment. The projection in the centre of 

 each valve, viz., the corpus Arantii, is better marked. Opposite each 

 valve the coat of the aorta is bulged as in the pulmonary artery, though 

 in a greater degree, and presents a little hollow on the inner side, named 

 sinus of Vahalva. The apertures of the coronary arteries are placed 

 behind two of the valves. 



Like the valves in the pulmonary artery these meet in the middle line 

 to stop the blood passing back into the ventricle, and combine with them 

 in causing the second sound of the heart. 



Position of the ventricular apertures. Two openings have been seen 

 in each ventricle one of the auricle of its own side of the heart, and one 

 of an artery. 



The apertures of the arteries (aorta and pulmonary) are nearest the 

 septum ; and as the two vessels were originally formed from one tube, 

 they are close together, but the pulmonary artery is the more anterior of 

 the two. The aperture of communication with each auricle is nearer the 

 circumference of the heart, and is posterior to the artery issuing from the 

 fore part of the ventricle. 



The position of the openings to one another from before backwards has 

 been before referred to, viz., the right is partly before the left auriculo- 

 ventricular; and the opening of the pulmonary artery is anterior to that 

 of the aorta, and rather higher than it. 



STRUCTURE. The heart is composed of strata of muscular fibres, and 

 of certain fibrous rings with a fibro-cartilage. 



The structure may be studied in the heart of the sheep or ox, in which 

 the fibres have been hardened and the connective tissue destroyed by boil- 

 ing, so as to allow of the fibres being separated. The description of the 

 structure of the heart may be omitted by the student till a suitable prepara- 

 tion of the fibres can be made. 



The fibrous structure forms rings around the auriculo-ventricular and 

 arterial orifices, and is prolonged into the valves connected with those 

 openings. 



The auriculo-ventricular rings give attachment to the framework of 

 fibrous tissue in the tricuspid and mitral valves. They are distinct from the 

 bands encircling the mouths of the arteries, except at the front of the left 

 auriculo-ventricular opening, where'the auricular and arterial circles blend. 



An arterial ring surrounds each large artery (aort i and pulmonary), 

 fixing those vessels, and giving attachment to some muscular fibres. It 

 is a circular band, with three notches in the margin towards the artery; 

 these are filled by corresponding projections of the artery, and give attach- 

 ment internally to the sigmoid valves along their semilunar edges. 



The artery is connected with the band of fibrous tissue in the following 

 manner: Its middle coat presents three projections, which are received 

 into the notches of the fibrous ring, being joined thereto by fibrous tissue; 

 and the junction between the two is strengthened externally by the outer 

 arterial coat and the pericardium, and internally by the endocardium. 

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