THE ARRANGEMENT OF THE MUSCULATURE OF THE HEART 263 



obvious that the circulation of the fetus greatly favors the head 

 region, the proper growth of the nervous system being of much greater 

 importance than that of the other tissues and organs. 



The distinctive features of the fetal system may, therefore, be 

 said to be the ductus venosus, the foramen ovale, the ductus arteriosus, 

 the hypogastric arteries, and the umbilical artery and vein. The 

 obliteration of these blood-vessels is initiated immediately after birth, 

 but several days usually elapse before this process has been completed. 

 Thus, the distal portions of the hypogastric arteries are usually found 

 to be impervious at the end of the third or fourth day, while the 

 obliteration of the ductus venosus and umbilical vein is not effected 

 until the end of the first week and that of the ductus arteriosus not 

 until the end of the third or fourth week. 



CHAPTER XXIV 



THE ARRANGEMENT OF THE MUSCULATURE OF THE HEART 

 THE VALVES OF THE HEART 



The Structure of the Auricles and Ventricles. — The adult human 

 heart measures about 125 mm. in length, 87 mm. in breadth, and 62 

 mm. in thickness. Its volume exhibits the following variations: 

 22 c.c. at birth, 155 c.c. during the fifteenth, 250 c.c. during the 

 twentieth, and 280 c.c. during the fiftieth year. Thus, it will be seen 

 that its growth is most rapid during early life. Beginning with about 

 the fifteenth year, the heart of the male becomes larger than that of 

 the female. At birth the organ weighs about 24 grams, at puberty 

 250 grams, and in adult life 310 grams. The heart of the adult female 

 weighs about 255 grams. To begin with, the ventricles are equally 

 heavy, but at the end of the second year the left weighs about twice 

 as much as the right, this relationship of 2 : 1 being retained until 

 death. 



The wall of the heart is composed of three layers, namely a lining 

 membrane, or endocardium, a median coat, or myocardium, and an outer 

 investment, or epicardium. The outermost layer forms at the same 

 time the inner or visceral half of the pericardium which is then reflected 

 from the base of the heart to serve as the parietal half of this capsular 

 investment. The space which is thus cut off from the general cavity 

 of the thorax, is known as the pericardial sac. Its opposing surfaces 

 are moistened with a few drops of a lymph-like fluid, called the peri- 

 cardial fluid. The function of the latter is to lessen the friction which 

 must necessarily be associated with the changes in the volume of the 

 heart. The pericardium contains many elastic fibers which coalesce 



