THE AORTA AND ITS BRANCHES. 



637 



The muscular fibres of the heart. The substance of the heart is 

 almost entirely composed of muscular fibres, arranged in layers, cov- 



Fig. 107. 



Fig. 107. a, the aorta, detached from the heart and from the 

 body. 1, ascending aorta, showing the enlargements or pouches 

 at its commencement, known as the sinuses of Valsalva; 1', 1"> 

 descending aorta, thoracic and abdominal; 2, 3, 3, 2, branches 

 from the arch of the aorta, which supply the head and upper 

 limbs ; 4, coeliac axis, or artery, which divides into three branches 

 to supply the stomach, liver, and spleen; 5, renal arteries, for 

 the kidneys; 6, superior, and 7, inferior mesenteric arteries, which 

 supply the small and large intestine; 8, 8, iliac arteries, which 

 give off branches to the pelvis and lower limbs ; 9, 9', intercostal 

 and lumbar arteries, which supply the walls of the thorax and 

 abdomen, b, portion of the left ventricle, and the commence- 

 ment of the aorta, laid open, to show the aortic semilunar valves; 

 1, portion of the aorta, with the orifices of the coronary arteries, 

 or nutrient arteries of the heart; 2, the three valve segments ; 

 3, portion of the left ventricle, c, diagrams, to show the action 

 of the valves in the veins ; 1, the valves open, so that the blood 

 can pass on ; 2, valves closed, so that the backward flow of the 

 blood is arrested. 



ered externally by a reflection of the serous membrane of the pericar- 

 dium, and lined, within the cavities, by the endocardium. Four fibrous 

 rings also exist, viz., those around the two auriculo-ventricular open- 

 ings and the orifices of the pulmonary artery and aorta. Besides 

 this, the heart possesses proper arteries, capillaries, and veins, deep 

 and superficial lymphatic vessels, and numerous nerves and nervous 

 ganglia. 



The fibrous rings of the left side of the heart are, like the valves, 

 stronger than those of the right side. Those of the auriculo-ventricular 

 orifices give attachment to the segments of their respective valves, 

 and also to a few of the muscular fibres of the auricles and ventricles. 

 The rings surrounding the orifices of the pulmonary artery and aorta 

 present a smooth ventricular border, attached to the muscular substance 

 of the ventricles; and a deeply notched arterial border, with which the 

 pouches of Valsalva are connected. When the heart of an animal is 

 boiled, so as to harden its muscular, and to gelatinize its fibrous, por- 

 tions, the auricles and great bloodvessels may be easily separated from 

 the ventricles ; three apertures only are then seen in the muscular sub- 

 stance of the base of the ventricles, viz., one for the left auriculo-ven- 

 tricular opening, another for the orifice of the pulmonary artery, and 

 a third, the largest, common to the left auriculo-ventricular opening 

 and the aortic orifice; for the fibrous rings around these two latter are 



