7IO HUMAN ANATOMY. 



leaflets, subsequently undergoes partial atrophy and disappears from the flaps and 

 adjoining parts of the attached bands, the latter thereby being converted into the 

 fibrous chordae tendineae. 



Even before the longitudinal subdivision of the bulbus arteriosus occurs, the 

 junction of this tube with the primary ventricle is marked by four cushion-like thick- 

 enings that project from the interior of the bulb. These elevations, which consist of 

 immature connective tissue covered by endothelium, furnish the leaflets of the aortic 

 and pulmonary semilunar valves. The formation of the aortic septum within the 

 bulbus arteriosus begins some distance above the valve and immediately below the 

 origin of the right and left pulmonary arteries. From this point the partition gradu- 

 ally grows downward until it. encounters the elongated lateral pair of the original four 

 valve-cushions, of which one lies in front, one behind, and two at the sides of the bulb. 

 With the completion of the division of the bulbus arteriosus into the systemic and 

 pulmonary aortae, the septum cleaves the two lateral cushions, each of the resulting 

 valves being guarded by three leaflets so disposed that the original and undivided 

 flap of the pulmonary artery lies in front, and that of the aorta behind. The partial 

 rotation that later places the aortic valve behind and to the left of the pulmonary 

 brings about the disposition observed in the adult (page 700), in which the single 

 •leaflet of the aortic semilunar valve lies in front and that within the pulmonary artery 

 is behind. At first comparatively thick, the leaflets suffer partial absorption, whereby 

 they are converted into the membranous cusps that bound crescentic pouches, the 

 sinuses of Valsalva, which lie between the leaflets and the wall of the vessels. 



PRACTICAL CONSIDERATIONS : THE HEART. 



It is possible here only to indicate with great brevity certain changes in the 

 position of the heart which should be studied in connection with its relations. 



The apex beat, normally to be found about one inch below and two inches to the 

 sternal side of the left nipple, is due to the recoil of the left ventricle as it empties its 

 contents into the aorta, to the lengthening of that vessel as the blood enters it, to the 

 consequent straightening of the arch (carrying the heart forward), and to the absence 

 of any interposed lung-tissue over the "area of absolute dulness. " 



The apex beat (and usually the heart itself) is («) raised in cases of ascites, 

 tympanites, large abdominal tumors, and atrophic pulmonary conditions ; (<5) de- 

 pressed in aortic aneurism, mediastinal growths, pulmonary emphysema, pleural 

 effusion, and hypertrophy or dilatation of the left ventricle ; (r) displaced laterally 

 to the right by left pleural effusion, splenic tumors, hypertrophy of the right ventri- 

 cle, to the left by hepatic tumors, right pleural effusion, hypertrophy of the left ven- 

 tricle. The heart may be drawn to either side by contracting pleural adhesions. 

 As the area of absolute dulness — "superficial cardiac area" — corresponds to that 

 portion of the cardiac substance which is not separated by pulmonary tissue from 

 the thoracic wall, it follows that its extent varies inversely with the size or expansion 

 of the lungs. In emphysema the area of cardiac dulness may quite disappear ; in 

 the later stages of fibroid phthisis it may be much larger than normal. 



In relation to the anatomy of the valves and cavities of the heart, the sounds 

 produced by the passage of blood through them should be considered in connection 

 with at least a few of the modifications caused by the chief pathological changes that 

 affect that organ. It may be said here, for the sake of clearness, that they?;-^/* sound 

 occurs during the contraction of the ventricles, when the auriculo-ventricular open- 

 ings should be closed by the mitral and tricuspid valves and the aortic and pul- 

 monary orifices should be open, and that it is due to (a') the shutting of the valves, 

 and (b) the impulse of the apex against the thoracic wall, with possibly some addi- 

 tion from (c) the contraction of the walls of the ventricles, although this latter factor 

 is doubtful. 



The seco7id sozind occurs during the auriculo-ventricular dilatation, and is due to 

 the closure of the pulmonary and aortic semilunar valves caused by the recoil of the 

 blood-current brought about by the elastic coats of the aorta and pulmonary arteries. 



If a murmur heard over the chest is synchronous with the radial pulse (systolic), 

 it occurs during ventricular contraction, and is usually due either (a) to regurgita- 



