STKUCTUKE OF THE HEAKT. 879 



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 lie in the centres of the cells. Moreover, still more peculiar fibres, the fibres of 

 Purkinje, are found immediately beneath the subendocardial tissue. The fibres of Purkinje 

 are large cells which unite with one another 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- 

 section, and after special methods of preparation, it is possible to recognise many layers and 

 bundles, some of which are, however, probably artificially produced. 



In the atria the muscular fasciculi fall naturally into two groups : (a) superficial fibres 

 common to both atria ; (b) deep fibres special to each atrium. 



The superficial fibres are most numerous on the sterno-costal aspect and in the neighbourhood 

 of the coronary sulcus. They run transversely across the atria and a few of them dip into the 

 interatrial septum. 



The deep fibres are (1) looped fibres. The extremities of the looped fibres are attached to 

 the fibrous rings around the atrio-ventricular orifices and the fibres pass antero-posteriorly over 

 the atria. (2) Annular fibres which surround (a) the extremities of the large vessels which open 

 into the atria ; (b) the auricles ; (c) the fossa ovalis. 



In the ventricles the muscular fasciculi form more or less definite V-shaped loops which 



, commence from and end at the fibrous rings which surround the large orifices at the bases of the 



ventricles. In their courses the loops embrace the cavities of either one or both ventricles, one 



stem of each loop lying on the outer surface of the heart and the other in the interior, and some 



of the loops possessing very acute whilst others have very open bends. 



The superficial fibres on the sterno-costal surface pass towards the left, those on the inferior 

 surface towards the right. At the apex all are coiled into a whorl or vortex through which they 

 pass into the interior of the ventricular walls and run towards the base, some in the septum 

 and others in the papillary muscles. The various bundles which have been described can, 

 according to Mall, be resolved into two main systems. One system arises from the conus 

 arteriosus and the root of the aorta, that is from the remains of the primitive aortic trunk : 

 ; it is called the " bulbo-spiral" system. The other springs from the region of the primitive 

 i venous sinus and is termed the " sino-spiral." Both systems are separable into superficial and 

 deep portions, and the general plan of more or less spirally curved V -sna P e( i loops is retained 

 in each, but the details of the arrangement are too complicated for consideration within the 

 limits of an ordinary text-book (see Amer. Journ. of Anat. vol. ii. 1910-1911). 



The Atrio-Ventricular Bundle. It would appear from the preceding description that the 



i muscle-fibres of the atria and the ventricles are entirely separated from one another by the fibrous 



rings which surround the atrio-ventricular orifices ; that, however, is not the case, for the two 



groups are connected together by a bundle of muscle fibres of pale colour and rudimentary 



| structure, which lies immediately adjacent to the endocardium and constitutes the atrio-ventricular 



. bundle. 



The bundle commences in a nodular enlargement which lies in the lower part of the wall 



; of the right atrium, close to the base of the medial cusp of the tricuspid valve. From that point 



the bundle runs along the posterior and lower borders of the membranous part of the ventricular 



septum to the upper and posterior part of the muscular portion of the septum, where it divides 



into right and left branches. The right branch runs along the right side of the ventricular 



septum to the moderator band, along which it passes into the anterior papillary muscle of the 



; right ventricle. The left branch runs along the left side of the septum and both branches give 



off numerous ramifications, by means of which the main bundle is associated with all parts of 



, the walls of the two ventricles. 



Both the function and the origin of the atrio-ventricular bundle are uncertain. After the 

 scovery of the bundle it was asserted that impulses were conveyed from the atria to the 

 ventricles by the muscle fibres of the atrio-ventricular bundle and by them alone ; more recently 

 t has been shown that minute nerve fibrils are intimately intermingled with the muscle fibres of 

 the bundle, and it has yet to be decided whether the impulses which pass from the atria to 

 the ventricles, for the purpose of maintaining the proper sequence of the movements of the 

 chambers, travel by the nerve fibrils or the muscle fibres or by both. 



The atrio- ventricular bundle is probably a remnant of the muscular continuity which 

 sted in the early stages of development between the atrial and ventricular chambers of the 

 : heart, but it may be, wholly or in part, a new formation. 



The epicardium, or visceral portion of the pericardium, consists of white connective and 



elastic tissue, the latter forming a distinct reticulum in the deeper part. The surface which 



looks towards the pericardial cavity is covered with flat polygonal endothelial plates, which are 



partially separated, here and there, by stomata. It has been asserted that the pericardial cavity 



communicates with the lymphatics of the epicardium through the stomata. 



The endocardium lines the cardiac cavities and is continuous with the inner coats of the 



s which enter and leave the heart. It consists, like the epicardium, of white connective 



5 and elastic fibres, but it is much thinner than the epicardium, and its elastic fibres are in 



some places blended into a fenestrated membrane. Its inner surface is covered with endothelial 



Us, and it rests externally upon the subendocardial tissue, in which there are blood-vessels and 



nerves ; the endocardium itself is entirely devoid of vessels. 



