HEART 139 



wall mapped out above, and is divided into an upper and lower 

 part by the auriculo- ventricular groove. At this point are the 

 orifices of the pulmonary artery and aorta from the right and 

 left ventricles respectively. 



The Interior of the Heart. Internally the heart is com- 

 pletely divided into right and left halves by. a septum indicated 

 on the outside by the longitudinal grooves. The transverse 

 septum is not complete, as each auricle communicates with the 

 ventricle below by a valvular opening. 



The Auricles are almost cubicle in form and each possesses a 

 well-marked process from its upper angle, which projects for- 

 wards on the outside of the heart, known as the auricular ap- 

 pendix. The walls are lined with a serous membrane the 

 endocardium and are smooth except in the region of the 

 appendix, where there are a few muscular fibres, known as 

 the musculi pectinati. The right auricle receives posteriorly the 

 .superior vena cava above and the inferior vena cava below, the 

 orifice of the latter being guarded by the rudimentary Eus- 

 tachian valve. The left auricle resembles the right' auricle, 

 except that it has four openings in its posterior wall the 

 orifices of the pulmonary veins. 



The Ventricles are conical in form and very muscular, the 

 left ventricle being larger and thicker-walled than the right. 

 The longitudinal septum is so placed that the apex of the heart 

 is formed entirely by the left ventricle. The right ventricle is 

 continuous with the right auricle, its base giving origin to the 

 pulmonary artery. The right auriculo- ventricular orifice is 

 guarded by the tricuspid valve, and the orifice of the pulmonary 

 artery is also guarded by a valve composed of three cusps. The 

 left ventricle is continuous with the left auricle, its base giving 

 origin to the aorta. The left auriculo-ventricular orifice is 

 guarded by the mitral or bicuspid valve, and the orifice of the 

 aorta by the aortic valve composed of three cusps. 



The cavities of the ventricles are lined with endocardium, 

 and are smooth except for the muscle fibres, which are more 

 numerous and more pronounced than those of the auricles. 

 Each cavity can be divided into two the body of the ventricle 

 and the vestibule of the vessel to which it gives origin. 



