KELATIOX OF I'AETS OF IIEAET TO WALL OF THOEAX. 253 



and left). From the anterior arises tlie right coronary artery ; from 

 the left posterior the left coronary artery : these vessels being for the 

 supply of blood to the snbstance of the heart. 



The capacity of the sinnses of Valsalva is greater, and the tendinons 

 tissue in the valves is more strongly marked at the mouth of the aorta 

 than at the connnencement of the pulmonary artery. 



During the contraction of the ventricle the valves lie at first against 

 the sides of the artery, and allow the blood to flow freely past them ; 

 but when the column of fluid in the artery is partially thrown back 

 by the elasticity of the coats of that vessel, the sigmoid valves are 

 floated back by the refluent blood, and completely close the arterial 

 orifice. When the valves are thus closed, the whole free border and 

 the thin Innated parts are closely applied to each other, and are held 

 together, as well as exempted from strain, by the opposite and equal 

 pressure of the blood on either side, so tliat the greater the pressure 

 the more accurate must be the closure. The force of the reflux is sus- 

 tained by the stouter and more tendinous part of the valve. 



The part of the ventricle adjoining the root of the aorta forms a 

 small compartment, the " aorfic vesiihuk " of Sibson, the walls of which 

 are fibrous, or, in some parts, fibro-cartilaginous. so that it remains 

 uncollapsed, and allows space for the liulging flaps of the aortic valve 

 to descend during diastole, besides allowing for the closure of the 

 mitral valve during extreme contraction of the ventricle. 



POSITION OP THE PARTS OF THE HEART WITH RELATION TO THE WAIiL 

 OF THE THOPuAX. 



The following statements are derived mainly from the observations 

 of Luschka and Allen Thomson : they have l)ceu carefully compared 

 with, and, where necessary, modified fi-om those of other observers,* 



Nearly two-thirds of the bulk of the heart lie to the left of the middle 

 line (fig, 174). The vpprr edije of the auricles corresponds with a line 

 extending across the sternum from the second right into the first left 

 intercostal space. The rinht auricle (3') extends laterally from about an 

 inch beyond the^ right border of the sternum to the middle of the left 

 half of that bone, and vertically fi'om the middle of the second right 

 cartilage to the lower border of the fourtli. The point of the rigid auri- 

 cular appendage is exactly behind the middle line on a level with the 

 upper border of the third costal cartilages (3), The left auricle 

 extends vertically from the level of the second left intercostal space 

 to the npper border of the fourth left cartilage ; and in breadth 

 corresponds to the body of the eighth dorsal vertebra and the head of the 

 adjoining rib. The apex of the left cmricidar appeiulage (4) is in the 

 lower part of the second intercostal space or behind the third costal 

 cartilage, about an inch and a quarter from the left of the sternum, 



* Luschka, Die Brustorgane, 1857; and Anatomie des Jlenschen, &c., 1863 ; Walslie, 

 Diseases of the Heart and Great Vessels ; Sibson, On tlie Noi-raal and Abnormal Situation 

 and Structure of the Viscera of tlie Chest, in Trans, of the Provinc. ]\Ied. and Surg. 

 Assoc, vol. xii., year 1842, and in his Work on Medical Anatomy, also article on the 

 Position and Form of the Heart, in RejTiolds' System of Medicine, vol. iv. ; Allen Thomson, 

 Notice of the case of E. Groux, &c., with Observations on the Position and Actions of 

 the Heart, in Grlasgow Med. Journ., April, 185S ; Pirogoff, Anatomia Topographica ; 

 Tiraune, Topographisch-Anatomischer Atlas ; Le Gendre, Anatomie Chinirgicale Homo- 

 logi-aphique. The three last-mentioned publications give the results obtained by meana 

 of sections of the chest at dififerent levels, made whilst frozen. 



