218 THE ACTION OF THE HEART. 



strengthen them, which are spoken of as the muscuh pec- 

 tonati. The valves are strong membranous appendages ; 

 and there are valves also to the commencement of each 

 artery, as well as between the auricles and ventricles on 

 each side. The valves to the pulmonary artery and the 

 aorta are called semilunar ; the arterial trunk being indented 

 behind each of the three flaps. The indentations are named 

 the sinus valsalvae. Upon the central portion of each flap 

 may be felt a little rounded body, known as the corpuscula 

 arantii. The use of these valves is to prevent the regurgi- 

 tation of the blood. The aorta never being quite empty, 

 when the ventricle dilates, there is a tendency to suck back 

 the amount of blood retained within the vessel ; but the 

 attempt to enter the ventricle pushes down the semilunar 

 valves, and shuts out its ingress, the corpusculi arantii 

 securing the middle space, which would be naturally left 

 between three semicircles essaying to close a circle. The 

 valves upon the right and left auriculo ventricular openings 

 have different names. That on the right consisting of three 

 flaps, being termed the tricuspid, and that of the left formed 

 by two flaps, being called the mitral. The heart is fur- 

 nished wdth blood for its own individual support by the 

 coronary arteries {Plate VIII. n. Fig 13 . 3), which is re- 

 turned by the coronary veins. Its nerves are gained from 

 the cardiac plexus, principally formed by the par vagum 

 and sympathetic nerves. 



THE PHYSIOLOGY OF THE ACTION OF THE HEART, AS THE 

 GRAND AGENT IN THE CIRCULATION OF THE BLOOD. 



The circulatory round of the blood is a vital action per- 

 formed by the heart. The round of the blood is divided 

 into the pulmonic and the general circulation. 



The pulmonic circulation. — The blood of the two cavas is 

 poured into the right auricle when that cavity dilates. It 

 is sent into the right ventricle wJien the right auricle con- 

 tracts. The right ventricle urges it into the pulmonary 

 artery [Fig 13 . 2), from w4iich tube it is prevented return- 

 ing by the valves at the origin of the vessel. By the divi- 

 sions of the pulmonary artery within the lungs the blood is 

 carried to the air cells ; and in the capillaries of the vessel 

 it is changed from venous into arterial blood, or becomes 



