878 THE VASCULAK SYSTEM. 



right of its communication with the atrium it is continued into the ascending 

 aorta. 



The mitral orifice is oval; its long axis runs obliquely from above down- 

 wards and to the right, and -it is guarded by a valve consisting of two cusps, 

 which is known as the bicuspid or mitral valve. The two cusps of the valve 

 are triangular and of unequal size. The smaller of the two is placed to the left 

 and inferiorly; and the larger, placed to the right and anteriorly, between the 

 mitral and aortic orifices, is known as the aortic cusp. The bases of the cusps 

 are either continuous with one another, at their attachments to the fibrous ring 

 around the mitral orifice, or they are separated by small intermediate cusps of 

 irregular form and size. The apices of the cusps project into the cavity of the 

 ventricle. The atrial surfaces are smooth ; the ventricular surfaces are roughened 

 by the attachments of the chordae tendinese, which are connected also with the 

 irregular and notched margins and with the apices. The structure is the same 

 as that of the cusps of the tricuspid valve, but the ventricular surface of the 

 anterior cusp is relatively smooth ; therefore the blood flow into the aorta is 

 not impeded. 



The aortic orifice is circular ; it lies immediately anterior and to the right of 

 the mitral orifice, from which it is separated by the anterior cusp of the mitral 

 valve, and it is guarded by the aortic valve, formed of three semilunar segments, 

 one of which is placed anteriorly and the other two posteriorly. The structure 

 and attachments of the cusps of the aortic valve are similar to those of the 

 cusps of the pulmonary valve (see p. 877). 



The cavity of the left ventricle is separable, like that of the right, into two 

 portions, the body and the aortic vestibule ; the latter is a small section placed 

 immediately below the aortic orifice ; its walls are non-contractile, consisting of 

 fibrous and fibro - cartilaginous tissue. The wall of the cavity is lined by 

 endocardium. . The inferior wall and the apex are rendered sponge-like by numerous 

 fine trabeculse carneae, whilst the upper part of the sterno-costal wall and the 

 septum are relatively smooth. 



There are two papillary muscles of much larger size than those met with in the 

 right ventricle an anterior and an inferior ; each is connected by chordae tendinese 

 with both cusps of the mitral valve. 



The walls of the left ventricle are three times as thick as those of the right 

 ventricle, and they are thickest in the region of the widest portion of the cavity, 

 which is situated about a fourth of its length from the base. The muscular portion 

 of the wall is thinnest at the apex, but the thinnest portion of the boundary lies at 

 the upper part of the septum, and it consists entirely of fibrous tissue ; this part 

 is occasionally deficient, and an aperture is left through which the cavities of the 

 two ventricles communicate. 



The ventricular septum is a musculo-membranous partition. It is placed 

 obliquely, so that one surface looks anteriorly and to the right, and bulges into the 

 right ventricle, whilst the other looks posteriorly -and to the left and is concave 

 towards the left ventricle. Its sterno - costal and inferior margins correspond 

 respectively with the anterior and the inferior portions of the interventriculai 

 sulcus, and it extends from the right of the apex to the interval between the 

 pulmonary, the aortic, and the atrio- ventricular orifices. The main part of ife 

 extent is muscular, and is developed from the wall of the ventricular part of tht 

 heart ; but its upper and posterior portion, the pars membranacea, which is developec 

 from the septum of the truncus arteriosus, is entirely fibrous, and constitutes th< 

 thinnest portion of the ventricular walls. The pars membranacea lies between th< 

 aortic vestibule of the left ventricle, on the left, and the upper part of the righ 

 ventricle and the lower and left part of the right atrium, on the right. 



STKUCTUEE OF THE HEAKT. 



The walls of the heart consist mainly of peculiar striped muscle, the myocardium, which i 

 enclosed between the visceral layer of the pericardium, or epicardium, externally, and th 

 endocardium internally. The muscular fibres differ from those of ordinary voluntary stripe 



