SYSTEM.] ANATOMY 901 orifices are without valves. At the lower part of tha auricle is the large orifice of communication between it and the base of the left ventricle. The Left Ventricle forms the left border, the apex, a large part of the posterior surface, but only a small part of the anterior surface of the heart. It is conical in form, its apex is at the apex of the heart, the base at the corre sponding auricle. As in the right ventricle, the inner surface of its wall is elevated into fleshy columns, two of which project like nipples into the cavity and form musculi papillarfs, which have chordce tendinece connected with them. The left auriculo- ventricular opening is large enough to admit 4wo fingers. It possesses a valve, which consists of two large pointed segments or cusps, between which two Email intermediate cusps lie, hence it is called the bicuspid valve ; and as these cusps are placed one in front of the other like the segments of a bishop s mitre, the name mitral valve is often given to it. The cusps agree in shape, general arrangement, and mode of attachment with those of the tricuspid valve, but they are stronger; and as the more anterior segment lies obliquely between the auricular and aortic orifices, both its surfaces are smooth. From the base of this ventricle the great systemic artery or aorta arises through the intermediation of a strong fibrous ring. The mouth of the aorta is surrounded by a three segmented semilunar valve, similar to the semilunar pulmonary valve, but with thicker and stronger segments, and possessing more strongly marked sinuses of Valsalva. The base of each ventricle has therefore two openings in it, one for communication with the auricle, the other with the great artery arising from the ventricle. The auriculo-ventricular openings are the most posterior, and almost in the same plane; the aortic opening lies in front of the interval between the two auriculo-ventricular, and the pulmonary opening is in front of the aortic. The walls of the cavities of the heart are formed of striped muscular fibre, over the contractions of which the will exercises no control. The fibres are collected into fasciculi, which have a reticulated arrangement, and the fibres themselves branch and again unite to form a compli cated network. The fibres of the walls of the auricles are distinct from those of the ventricles, so that the auricular and ventricular compartments are connected together, not by an interchange of muscular tissue, but by an intermediate ring-like arrangement of fibres of connective tissue. The muscular fasciculi of the auricles are arranged in two strata. The deeper stratum consists of fibres proper to each auricle, some of which run obliquely in the wall, others form the musculi pectinati, surround the auricula, and are prolonged in rings into the coats of the venae cavte and pulmonary veins, whilst fibres extend longitudinally and obliquely along the wall of the coronary venous sinus. The superficial stratum consists of fasciculi, which run obliquely from one auricle to the other on both the anterior and posterior surfaces, and are said to be prolonged into the auricular septum. The muscular wall of the ventricles is much thicker than that of the auricles, and the wall of the left ventricle is about three times thicker than the right. The fibres vary in their direction in different parts of the thickness of the ventricular walls. The superficial ex ternal fibres run obliquely from above downwards, and from right to left, and on the anterior surface of the ven tricles dip into the anterior ventricular groove to enter the septum, whilst on the posterior surface they extend across the posterior ventricular groove ; at the apex of the heart they turn inwards in a whorl-like manner, and, as was known to Lower and Gerdy, become continuous with superficial fibres on the inner wall of the ventricle ; at the base of the ventricles they turn round the border of the auriculo-ventricular openings, and, as Pettigrew lias shown, become continuous with these superficial internal fibres, which run in the reverse direction. The internal fibres are also prolonged into the musculi papiilares, the chords* tendinese springing from which serve therefore as tendons of insertion for these muscles. If the substance of the wall be now dissected the fibres situated in the centre of the wall are seen to lie in the horizontal plane. Various anatomists have described these fibres of the ventricles as arranged in layers. Lower recognised two layers spirally crossing each other ; Haller, three ; Wolff, three in the right and six in the left ventricle. Pettigrew at one time believed he could dissect nine layers, but has subsequently reduced the number to seven three external, a fourth or central, and three internal. He conceives that the fibres of the three external layers run in a spiral direction from left to right downwards, the first layer being more vertical than the second, and the second than the third, whilst the fibres of the fourth or central layer are horizontal. The three internal layers also run spirally, but in the reverse direction from the external, with which they become con tinuous both at the base and apex. The subdivision of the ventricular wall into such precise and determinate layers as is implied in the descriptions of Pettigrew is, however, to some extent an artificial procedure. There can be no doubt, as his dissections so beautifully show, that the direction of the fibres in the ventricular wall varies at different depths ; but owing to the reticulated arrangement of the fibres, not only are those connected together which lie in one of the so-called layers, but they also anastomose with the fibres in the layer contiguous to it on either aspect. Hence when one layer is peeled off, that imme diately subjacent exhibits, not a smooth face, which it would have done had the definition of the layers been dis tinct, but a rough appearance, due to the tearing through of intermediate connecting muscular fibres. Owing to these connections the substance of the wall of the ventricle, as Henle s dissections show, may, with the exception of the superficial internal and external fibres, be split up into lamellae, which extend either horizontally, obliquely, or in an arched manner through the wall between its two sur faces ; and the surfaces of those lamellae are not parallel to the wall of the ventricle, but are directed upwards and downwards. > Except at the fibrous rings, where both the white and yellow fibres are distinct, the connective tissue of the heart is small in quantity. The endocardial lining consists of connective tissue with elastic fibres, with a layer of endo- thelium on the free surface ; and Schweigger-Seidel has also described smooth muscular fibres in it. Hence, as Luschka has stated, the endocardium represents not merely the inner coat of the blood-vessels but all the structures of the vascular wall. Purkinje described fibres beneath the endocardium, which are now regarded as imperfectly formed striated muscular fibres. The valves are folds of the endo cardium, enclosing fibres continuous with those in the fibrous rings : the cuspidate auriculo-ventricular valves receive fibres from the chord?e tendines?. The heart is well supplied with blood, not by the blood which flows through its cavities, but by two special coro nary arteries which ramify in its walls, and end in nume rous capillaries lying between the fibres. From these capillaries the coronary veins arise, which join to form the coronary venous sinus. Lymphatic vessels occur both in the endocardium and pericardium, and apparently ramify in the muscular wall. The nerves of the heart have been dissected especially by Scarpa, Eemak, Lee, and Pettigrew, and numerous small ganglia described in connection with them (see p. 883).
The blood flows along the great veins into the auricles.
