THE HEART 571 



opening is behind the sternum, rather to the left of the median line, and opposite the fourth 

 costal cartilages. The right auriculoventricular opening is a little lower, opposite the fourth 

 interspace and in the middle line of the body (Fif,. 410). 



A portion of the area of the heart thus mapped out is uncovered by lung, and therefore gives 

 a dull note on percussion; the remainder, being overlapped by the lung, gives a more or less 

 resonant note. The former is known as the area of complete cardiac dulness. The area of 

 complete cardiac dulness is included between a line drawn from the centre of the sternum, on a 

 level with the fourth costal cartilage, to the apex of the heart and a line drawn from the same 

 point down the lower third of the mid-line of the sternum. Below, this area merges into the 

 dulness which corresponds to the liver. 



Applied Anatomy. Wounds of the heart are often immediately fatal, but not necessarily 

 so. They may be nonpenetrating, when death may occur from hemorrhage, if one of the coro- 

 narv vessels has been wounded, or subsequently from pericarditis; or, on the other hand, the 

 patient may recover. Even a penetrating wound is not necessarily fatal, if the wound is a small 

 one. An attempt should be made to save the patient by means of a surgical operation. A 

 trap-door flap comprising the whole thickness of the thoracic wall should be made. The hinges 

 of the trap-door are the rib cartilages. The pericardium is exposed and freely opened, clots 

 are removed, the wound in the heart is sought for, and when discovered is sutured. In a pene- 

 trating wound the sutures include the whole thickness of the heart, except the endocardium. 

 Interrupted sutures should be used, and each one had better be tied during diastole. A number 

 of successful operations of this character have been performed. 



Peculiarities in the Vascular System of the Fetus (Fig. 426). 



The chief peculiarities in the heart of the fetus are the direct communication 

 between the two auricles through the foramen ovale, and the large size of the 

 Eustachian valve. There are also several minor peculiarities. Thus, the position 

 of the heart is vertical until the fourth month, when it commences to assume an 

 oblique direction. Its size is also very considerable as compared with the body, 

 the proportion at the second month being 1 to 50; at birth it is as 1 to 120; while 

 in the adult the average is about 1 to 160. At an early period of fetal life the auric- 

 ular portion of the heart is larger than the ventricular, the right auricle being 

 more capacious than the left; but toward birth the ventricular portion becomes 

 the larger. The thickness of both ventricles is at first about equal, but toward 

 birth the left becomes much the thicker of the two. 



The foramen ovale (Fig. 425) is situated at the lower and back part of the auric- 

 ular septum, forming a communication between the auricles. It remains as a free 

 oval opening until the middle period of fetal life. About this period a fold grows 

 up from the posterior wall of the auricle to the left of the foramen ovale, and 

 advances over the opening so as to form a sort of valve, which allows the blood 

 to pass only from the right to the left auricle, but not in the opposite direction. 



The Eustachian valve (Fig. 425) projects upward in front of the opening of 

 the inferior vena cava, and tends to direct the blood from this vessel through the 

 foramen ovale into the left auricle. 



The peculiarities in the arterial system of the fetus are the communication 

 between the pulmonary artery and the descending aorta by means of the ductus 

 arteriosus, and the continuation of the internal iliac arteries as the umbilical 

 arteries to the placenta. 



The ductus arteriosus (Fig. 426) is a short tube, about 10 mm. (half an inch) in 

 length at birth, and 2 mm. (one-twelfth of an inch) in diameter. In the early 

 condition it forms the continuation of the pulmonary artery, and opens into the 

 descending aorta just below the origin of the left subclavian artery, and so con- 

 ducts the greater part of the blood from the right ventricle into this vessel. When 

 the branches of the pulmonary artery have become larger relatively to the ductus 

 arteriosus, the latter is chiefly connected to the left pulmonary artery; and the 

 fibrous cord (ligamentum arteriosum), which is all that remains of the ductus 

 arteriosus in later life, will be found to be attached to the root of that vessel. 

 Occasionally a small lumen persists in the ligamentum arteriosum. 



