746 THE VASCULAR SYSTEM. 
cusp. The bases of the cusps are either continuous with each other at their 
attachments to the fibrous ring round the mitral orifice, or they are separated by 
small intermediate cusps of irregular form and size. The apices of the cusps hang 
down into the cavity of the ventricle. The auricular surfaces are smooth; the 
ventricular surfaces are roughened by the attachments of the chorde tendinee, 
which are also connected 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 (or aortic) cusp is relatively smooth. 
The aortic orifice is circular; it les immediately in front 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 
(valvule semilunares aortz), one of which is placed anteriorly and the other two 
posteriorly. The structure of these cusps and their attachments are similar to 
those of the cusps of the pulmonary valve. 
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, and its walls are non-contractile, consisting of 
fibrous and fibro-cartilaginous tissue. The cavity is lied by endocardium. The 
inferior wall and the apex are rendered sponge-like by numerous fine columne 
varneve of the first and second classes, whilst the upper part of the antero-superior 
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 a posterior; each is connected by chord tendinex 
with both cusps of the mitral valve. 
The walls of the left ventricle, with the exception of the septum, 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 attains its minimum thickness at the 
apex, but the thinnest portion of the boundary is at the upper part of the septum, 
which consists entirely of fibrous tissue; here it is occasionally deficient, and an 
aperture is left through which the cavities of the two ventricles communicate. 
The interventricular septum (septum ventriculorum) is a musculo-membranous 
partition. It is placed obliquely, one surface looking forwards and to the right 
and bulging into the right ventricle, and the other backwards and to the left 
towards the left ventricle. Its antero-superior and inferior margins correspond 
respectively with the anterior and inferior portions of the interventricular sulcus, 
and it extends from the right of the apex to the interval between the pulmonary 
and aortic orifices. In the main part of its extent if is muscular (septum musculare 
ventriculorum), and this portion is developed from the wall of the ventricular part 
of the heart; but its upper and posterior portion, the pars membranacea (septum 
membranaceum ventriculorum), which is developed from the septum of the aortic 
bulb, is entirely fibrous, and constitutes the thinnest portion of the ventricular 
walls. The pars membranacea hes between the aortic vestibule on the left and 
the upper part of the right ventricle, as well as the lower and left part of the right 
auricle, on the right. 
STRUCTURE OF THE HEART. 
The walls of the heart consist mainly of peculiar striped muscle, the myocardium, which is 
enclosed between the visceral layer of the pericardium, or epicardium, externally, and the 
endocardium internally. The muscular fibres differ from those of ordinary voluntary striped 
muscle in several ways: they are shorter, many of them being oblong cells with forked ex- 
tremities which are closely cemented to similar processes of adjacent cells ; they form a reticulum, 
and the nuclei he in the centres of the cells. Moreover, in some of the lower mammals, in the 
young child up to the ‘end of the first year, and occasionally in the human adult also, still more 
peculiar fibres, the fibres of Purkinje, are found immediately beneath the sub-endocardial 
tissue. These are large cells which unite with each other at their extremities; their central 
portions consist of granular protoplasm, in which sometimes one, but more frequently two nuclei 
are embedded, and the peripheral portion of each cell is transversely striated. These cells, in short, 
present in a permanent form a condition which is transitory in all other striped muscle cells. 
The reticulating cardiac muscle cells are grouped in sheets and strands which have a more or 
less characteristic and definite arrangement in different parts of the heart; by careful dis- 
