82 



ANATOMY FOE, NUKSES. [CHAP. VIII. 



by a longitudinal partition into a right and a left heart, each of 

 which is subdivided by a transverse constriction into two com- 

 partments, an upper and a lower, which communicate with each 



other. Its general form 

 is that of a blunt cone. 

 It is situated in the 

 thorax, between the 

 lungs, and, together 

 with the adjacent parts 

 of the great blood-ves- 

 sels which carry blood 

 to and from it, is en- 

 closed in a membranous 

 covering, the pericar- 

 dium. The heart lies 

 nearer to the front 

 than to the back of the 

 chest, and is placed be- 



FIG. 63. THE HEART AND LUNGS. 1, right ven- , . , , , 



tricle; 3, right auricle; 6, 7, pulmonary artery; 9, nmd 

 aorta; 10, superior vena cava; 11, innominate ar- 

 tery ; 12, right subclavian vein ; 14, innominate vein ; 

 15, left common carotid; 17, trachea; 20, pulmonary 

 veins ; 22 to 25, lungs, partially turned back to show "base bein Q directed UD- 

 veins on left side. 



wards, backwards, and 



to the right, while the pointed end or apex points downwards, 

 forwards, and to the left. The impulse of the heart against the 

 wall of the chest is felt in the space between the fifth and sixth 

 ribs, a little below and to the inner side of the left nipple. It 

 has, therefore, a very oblique position in the chest. It is sus- 

 pended and kept in position by the great vessels at the base, 

 and is also supported by the diaphragm. According to Laennec, 

 the heart in its normal condition is about equal in size to the 

 fist of the individual to whom it belongs. 



The main substance of the heart is composed of muscular 

 tissue. Between the muscle fibres is a certain amount of in- 

 terstitial tissue with numerous blood-vessels and lymphatics, 

 and, in some parts, nerves and ganglia. There is also a consid- 

 erable amount of fat, chiefly collected at the base of the heart 

 and beneath the pericardium. The muscular tissue of the heart 

 differs from all other involuntary muscular tissue in possessing 

 transverse striae. The fibres continually branch and unite with 



costal cartilages, 

 broader end or 



