578 



HEART. 



theless communicate freely with each other. It 

 would appear, however, from Meckel's descrip- 

 tion, that the ventricle is divided by a complete 

 septum into two separate and distinct chambers 

 in the Crocoililus lucius. In the Mammalia 

 and Birds, where no intermixture of the venous 

 and arterialized blood takes place, but where 

 all the blood sent along the aorta lias been pre- 

 viously subjected freely to the influence of the 

 atmospheric air, we find two distinct hearts, 

 which in the adult have no communication 

 with each other; one the respiratory heart for 

 the transmission of the blood through the lungs, 

 the other the systemic heart for the transmission 

 of the arterialized blood through all the textures 

 of the body. These are not placed separate 

 from each other, as in some of the Mollusca, 

 which with a double circulation have an aquatic 

 respiration, but are in juxta-position, and in 

 fact many of the muscular fibres are common 

 to both. 



HUMAN HEART (normal anatomy). 

 Position. The heart in the human species 

 is lodged within the cavity of the thorax, occu- 

 pies the middle mediastinum, and is enclosed 

 in a fibro-serous capsule called pericardium. 

 It is placed obliquely from above downwards 

 and from behind forwards, in front of the spine 

 and behind the stenium. The apex is directed 

 downwards, forwards, and to the left side, pro- 

 jects into the notch on the anterior margin of 

 the left lung, and in the quiescent state of the 

 organ corresponds to the posterior surface of 

 the cartilage of the sixth rib. The base looks 

 upwards, backwards, and to the right side ; is 

 separated from the anterior part of the spine by 

 the pericardium, oesophagus, aorta, and other 

 parts which lie in the posterior mediastinum ; 

 and extends from about tlie fourth to the eighth 

 dorsal vertebra. Its right margin rests upon 

 the upper surface of the cordiform tendon of 

 the diaphragm, by which it is separated from 

 the stomach and liver ; its left margin, which 

 is more vertical, looks upwards and to the left 

 side, and occupies an excavation on the inner 

 surface of the left lung. Its posterior or flat 

 surface rests partly upon the cordiform tendon 

 of the diaphragm, having the pericardium inter- 

 posed between them, and partly upon the inner 

 concave surface of the left lung.* Its position 

 corresponds to the union of the superior third of 

 the body with the two inferior thirds. The lungs 

 overlap the lateral, and part (rarely the whole) 

 of the anterior portion of the heart, leaving 

 only in general about an inch and a half or 

 two square inches of the anterior surface of the 

 right ventricle uncovered by the lung. It is of 

 importance to remember this fact in percussing 

 this region. The two sacs of the pleura, as 

 they pass between the spine and sternum to 

 form the mediastina, are interposed between 

 the lungs and the heart. The heart is subject 

 to slight change of position from the influence 

 of the contiguous organs. It is carried a little 

 downwards durin" violent contraction of the 



* In the lower animals its position is vertical, 

 occupying the ntesiul line of the body. 



diaphragm, and is pressed upwards when the 

 abdominal viscera are strongly compressed by 

 the powerful contraction of the abdominal 

 muscles. During expiration it has been seen 

 to recede deeper into the thorax, and during 

 inspiration again to come forward, \\hen the 

 body is bent to the right side, the apex recedes 

 from the inner side of the left wall of the 

 thorax ; when bent to the left side, it is still 

 more closely approximated to it. 



l'\ii-m unit external surface. Its form is that 

 of a flattened cone, and it is neither symme- 

 trical as regards the mesial line of the body, 

 nor (as we shall afterwards find) is the organ 

 itself symmetrical. It presents an anterior and 

 a posterior surface; a right inferior or acute 

 margin ; a left superior or obtuse margin ; a 

 base, and an apex. Its anterior surface, which 

 is also turned towards the left side, is convex 

 and considerably longer than the posterior and 

 right, which is flattened. On the anterior sur- 

 face of the heart we find a distinct groove, 

 running nearly in the axis of the organ, passing 

 from above downwards and from right to left, 

 and containing the left coronary artery. A 

 larger portion of the heart appears to he to the 

 right than to the left of this groove. There is 

 a similar groove on the posterior surface, which 

 is nearly vertical, shorter than the anterior, and 

 contains a branch of the right coronary artery. 

 These two grooves are connected with each 

 other at or near the apex generally by a small 

 notch, which is sometimes of sufficient depth 

 to give the heart a bifid appearance.* These 

 grooves mark the division of the heart into 

 right and left sides. These terms are, how- 

 ever, more applicable when describing the 

 organ in the lower animals ; for in the human 

 species the right side is also anterior and infe- 

 rior, and the left side posterior and superior. 

 Near the base of the heart and at the com- 

 mencement of the longitudinal grooves, we find 

 a circular groove deeper anteriorly than poste- 

 riorly, which contains in its posterior part the 

 coronary vein and branches of the coronary 

 arteries. This circular groove points out the 

 division between the auricular and ventricular 

 portions of the heart. Two large arteries are 

 placed in front of the anterior part of this 

 groove, the one posterior to the other. That 

 nearest to the groove is the aorta, which springs 

 from the base of the left ventricle ; the one 

 placed anterior is the pulmonary artery, which 

 arises from the upper part of the right ventricle, 

 and at its origin covers, along with that part of 

 the ventricle to which it is attached, the com- 

 mencement of the aorta. The ventricles form 

 the principal part of the heart, and occupy the 

 middle and apex, while the auricles are placed 

 at the base. The base of the ventricles is con- 

 nected to the base of the auricles. Two large 

 veins, the superior and inferior venae cavae, 



* This notch in the human heart looks like the 

 rudiments of the fissure which in the Dugong and 

 Rytina separates the two ventricles from each other 

 nearly up to the base. This bind form of the heart, 

 which is merely a temporary condition in the hu- 

 man species, is permanent in the Dugong and Ry- 

 tina. See Jig. 2(54, vol. i. p. 576. 



