THE CHAMBERS OF THE HEART. 693 



were it not that the course of the blood passing through the two orifices is in opposite 

 directions, and the auscultation of the auriculo-ventricular orifice is consequently 

 satisfactorily performed towards the apex of the heart. 



Considerable variation from the position of the heart indicated above may be found. Thus, 

 the apex may be situated behind the fifth costal cartilage, or more rarely the sixth, and the pul- 

 monary aortic orifice may occur as high up as the second intercostal space, or as low as the level 

 of the fourth costal cartilage. 



The heart naturally has its position altered somewhat during its contraction and during the 

 respiratory acts, and the position of the body will also have some effect in modifying its location. 

 Resting, as it does, upon the diaphragm, the heart will alter its position somewhat with altera- 

 tions of that muscle ; and since in the child the diaphragm is somewhat higher and in the aged 

 somewhat lower than in the middle period of life, corresponding changes according to age will 

 be found in the position of the heart. It may be noted, furthermore, that the position of the 

 heart as determined in the cadaver will, as a rule, be slightly higher than in the living body, 

 owing to post-mortem tissue changes which allow the diaphragm to assume a more vaulted form 

 than is usual in life. 



Relations. — As regards its relations the heart is completely enclosed within the 

 pericardium, with which alone surrounding organs come into contact. In what fol- 

 lows it is really the relations of the pericardium that will be described, although of 

 necessity these relations are indirectly those of the heart and will be spoken of as 

 such. 



Aiiteriorly the greater part of the heart is covered by the anterior borders of 

 the lungs and pleurse, which separate it from contact with the anterior thoracic wall. 

 As a rule, the anterior borders of the pleurce are in contact from the level of the 

 second costal cartilage to that of the fourth, but below the latter level they separate,' 

 the border of the left pleura diverging from the median line more rapidly than that 

 of the right. In consequence, throughout an irregularly triangular area (Fig. 1580), 

 whose vertical diameter extends from the level of the fourth to that of the sixth 

 costal cartilages, the heart is uncovered by the pleurae and lies directly behind the 

 thoracic wall. This area forms what is termed by clinicians the area of absolute dul- 

 ness. Laterally the heart is in relation with the lungs, the phrenic nerves passing 

 downward on either side between the pericardium and the pleura. Posfe?'io?iy the 

 relations are again with the lungs and with the oesophagus and the thoracic aorta. 

 Inferiorly the heart rests directly upon the diaphragm, beneath which is the 

 stomach. 



Size and Weight. — ^There is considerable individual variation in the size of 

 the heart, and marked discrepancies exist in the observations that have been re- 

 corded. It may be said that in the adult the heart, on an average, will possess a 

 length of from 12-15 cm. (434-6 in.), a greatest breadth of from 9-1 1 cm. 

 (3)^-4^ in.) and a thickness of from 5-8 cm. (2-3 J^ in.). 



Its weight has been given at from 266-346 gm. (9^-12^ oz.) for males and 

 from 230-340 gm. (8y^-i2 oz. ) for females, the average of a series of observations by 

 different authors giving 312 gm. ( 1 1 oz. ) for the male and 274 gm. (93^ oz. ) for the 

 female. The proportion of heart to the weight of the entire body, according to an 

 average drawn from several observers, is i : 169 in the male and i : 162 in the female. 

 It must be remembered, however, that the weight of the heart increases with age up 

 to about the seventieth year, probably a slight diminution taking place after that 

 period. 



THE CHAMBERS OF THE HEART. 



It has already been noted that the heart is composed of four chambers, a right 

 and left auricle and a right and left ventricle. As the heart lies in position, little of 

 the auricles, with the exception of the auricular appendices, can be seen, since they 

 have m front of them the roots of the aortae. In the ventricular portion the greater 

 part of the anterior surface is formed bv the right ventricle, a small portion only 

 of the left ventricle showing to the left and at the apex, the whole of which is formed 

 by the left ventricle. The four chambers will now be considered in succession, begin- 

 nmg with the auricles. 



The Right Auricle.— The right auricle ratrium dextnim) is a relatively thin- 

 walled chamber having in cross-section a roughly triangular form, the various sur- 



