THE THORAX 985 



lather hand, when the aortic valve is incompetent, and allows regurgitation 

 6i blood to take place into the left ventricle, the cardiac affection is called 

 aortic regurgitation (incompetence). 



The pulmonary valve, which guards the orifice of the pulmonary 

 artery, is similar to the aortic valve, and the preceding description 

 is, for the most part, applicable to it. The segments of the pul- 

 monary valve are, however, weaker than those of the aortic valve, 

 and the sinuses of Valsalva are destitute of any arterial orifices. 

 These sinuses are disposed as anterior, right postero-lateral, and lejt 

 Postero -lateral, respectively. 



The function of the pulmonary valve is similar to that of the 

 aortic valve. 



Interventricular Septum. — This septum completely separates the 

 right and left ventricles. The external indications of its attach- 

 ments are the antero-superior and inferior interventricular grooves. 

 It is thick and, for the most part, fleshy. It is thickest in the 

 region of the apex, and becomes thinner towards the base of the 

 ventricles. It bulges into the right ventricle, so as to be convex 

 on that aspect, whilst it recedes from the left ventricle, so as to be 

 concave towards it. The upper or basal part of the septum pre- 

 sents anteriorly a small portion which is destitute of muscular 

 fibres. This portion is thin, and consists of fibrous tissue, covered 

 on either side by endocardium. It is known as the pars mem- 

 branacea septi, and is developed from the lower part of the septum 

 which divides the aortic bulb into the ascending aorta and trunk 

 of the pulmonary artery. It forms part of the aortic vestibule of 

 the left ventricle. In rare cases the interventricular septum is 

 deficient in the region of the pars membranacea, and a communica- 

 tion thus exists between the right and left ventricles. 



Topography of the Orifices of the Heart. — The pulmonary orifice 

 is situated on a level with the upper margin of the third left costal 

 cartilage at its junction with the sternum. The aortic orifice, 

 which is more deeply placed than the pulmonary, is situated 

 behind the left border of the sternum on a level with the lower 

 margin of the third left costal cartilage. 



The tricuspid orifice lies behind the body of the sternum opposite 

 the fourth intercostal spaces and fourth and fifth costal cartilages. 

 The mitral orifice, which is placed very deeply, is situated behind 

 the left border of the sternum on a level with the fourth left costal 

 cartilage. 



Cardiac Bloodvessels — Arteries. — ^The nutrient vessels of the 

 heart are the coronary arteries, right and left, which arise from the 

 root of the ascending aorta. For a description of these vessels, 

 see p. 958r 



Veins. — The cardiac veins are as follows : the great or left cardiac 

 or coronary vein ; the coronary sinus ; the posterior cardiac veins ; 

 the middle cardiac vein ; the right or small cardiac or coronary 

 vein ; the anterior cardiac veins ; the oblique vein of Marshall ; 

 and the vense minimae cordis or smallest cardiac veins. With the 



