1096 THE THORAX. 



the great, the middle, anterior, and posterior, cardiac veins, the right or small and the 

 left or great coronary sinuses, and the venae cordis minimae (vence Thebesii) (p. 677). 



The lymphatics terminate in the thoracic and right lymphatic ducts. 



The nerves are derived from the cardiac plexuses, which are formed partly from 

 the cranial nerves and partly from the sympathetic. They are freely distributed 

 both on the surface and in the substance of the heart, the separate filaments being 

 furnished with small ganglia. 



Surface Form. In order to show the extent of the heart in relation to the front of the 

 chest, draw a line from the lower border of the second left costal cartilage, one inch from the 

 sternum, to the upper border of the third right costal cartilage, half an inch from the sternum. 

 This represents the base-line or upper limit of the organ. Take a point an inch and a half 

 below and three-quarters of an inch internal to the left nipple that is, about three and a half 

 inches to the left of the median line of the body. This represents the apex of the heart. 

 Draw a line from this apex-point, with a slight convexity downward, to the junction of the 

 seventh right costal cartilage to the sternum. This represents the lower limit of the heart. 

 Join the right extremity of the first line that is, the base-line with the right extremity of 

 this line that is, to the seventh rigbt chondro-sternal joint with a slight curve outward, so 

 that it projects about an inch and a half from the middle line of the sternum. Lastly, join the 

 left extremity of the base-line and the apex-point by a line curved slightly to the left. 



The position of the various orifices is as follows : viz. the pulmonary orifice is situated in 

 the upper angle formed by the articulation of tbe third left costal cartilage with the sternum ; 

 the aortic orifice is a little below and internal to this, behind the left border of the sternum, 

 close to the articulation of the third left costal cartilage to this bone. The left auriculo-yentric- 

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

 fourth costal cartilages. The right auriculo- ventricular opening is a little lower, opposite the 

 fourth interspace and in the middle line of the body. 



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

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

 less resonant note. The former is known as the area of superficial cardiac dulness ; the latter 

 as the area of deep cardiac dulness. The area of superficial cardiac dulness is included between 

 a line drawn from the centre of tbe sternum, between the fourth costal cartilages, to the apex 

 of the heart and a line drawn from the same point down tbe lower third of the middle line of 

 the sternum. Below, this area merges into the dulness which corresponds to the liver. Dr. 

 Latham lays down the following rule as a sufficient practical guide for the definition of the por- 

 tion of the heart which is uncovered by lung or pleura : " Make a circle of two inches in diam- 

 eter round a point midway between the nipple and the end of the sternum, ' ' that is, the gladiolus. 



Peculiarities in the Vascular System of the Foetus. 



The chief peculiarities in the heart of the foetus 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 ; whilst 

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

 auricular 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 is situated at the lower and back part of the septum auricu- 

 larum, forming a communication between the auricles. It remains as a free oval 

 opening from the time of the formation of the auricular septum (about the eighth 

 week) until the middle period of fcetal 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, and not in the opposite direction. 



The Eustachian valve is developed from the anterior border of the inferior vena 

 cava at its entrance into the auricle. It is directed upward on the left side of the 

 opening of this vein, and serves to direct the blood from the inferior vena cava 

 through the foramen ovale into the left auricle. 



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

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



