ARRANGEMENT OF THE VENTRICULAR FIBRES. 



53 



into the auricle ; and by their presence and action they prevent any large quantity 

 of blood from passing backward into the veins when the auricle contracts. [No 

 valves are present in the superior and inferior vena cava in the adult heart, or in 

 the pulmonary veins, hence the contraction of these circular muscular fibres plays 

 an important part in preventing any reflux of blood during the contraction of the 

 auricles.] 



45. ARRANGEMENT OF THE VENTRICULAR FIBRES. (2) The 



muscular fibres in the thick wall of the ventricles are arranged in several layers 

 (fig. 28, A) under the pericardium. First, there is an outer longitudinal layer (A) 

 which is in the form of single bundles on the right ventricle, but forms a complete 

 layer on the left ventricle, where it measures about one-eighth of the thickness of 

 the ventricular wall. A second longitudinal layer of fibres lies on the inner surface 

 of the ventricles, distinctly visible at the orifices, and within the vertically placed 

 papillary muscles, whilst elsewhere it is replaced by the irregularly arranged 

 trabecular carnese. Between these two layers there lies the thickest layer, consisting 

 of more or less transversely arranged bundles, which may be broken up into single 

 layers more or less circularly disposed. The deep lymphatic vessels run between the 

 layers, whilst the blood-vessels lie within the substance of the layers, and are sur- 

 rounded by the primitive bundles of muscular fibres. All three layers are not 

 completely independent of each other; on the contrary, the fibres which run 

 obliquely form a gradual transition between the transverse layers and the inner and 

 outer longitudinal layers. It is not, however, quite correct to assume that the 



Fig. 28. 



Course of the ventricular muscular fibres. A, on the anterior surface ; B, view of the apex with 

 the vortex ; C, course of the . fibres within the ventricular wall ; D, fibres passing into a 

 papillary muscle. 



outer longitudinal layer gradually passes into the transverse, and this again into 

 the inner longitudinal layer (as is shown schematically in C) ; because, as Henle 

 pointed out, the transverse fibres are relatively far greater in amount. In general, 

 the outer longitudinal fibres are so arranged as to cross the inner longitudinal layer 

 at an acute angle. The transverse layers lying between these two form gradual 

 transitions between these directions. At the apex of the left ventricle, the outer 

 longitudinal fibres bend or curve so as to meet at the so-called vortex B, where they 

 enter the muscular substance, and, taking an upward and inward direction, reach 

 the papillary muscles, p, D ; although it is a mistake to say that all the bundles 

 which ascend to the papillary muscles arise from the vertical fibres of the outer 

 surface : many seem to arise independently within the ventricular wall. According 

 to Henle, all the external longitudinal fibres do not arise from the fibrous rings or 

 the roots of the arteries. The mitral orifice is surrounded by circular fibres which 

 act like a sphincter {Henle). 



