ARRANGEMENT OF THE VENTRICULAR FIBRES. 69 



pulsate. The auricular portion of the right auricle beats longest ; 

 hence, it is called the " ultimum moriens." Independent rhythmical 

 contractions of the venae cavae and pulmonary veins are often noticed 

 after the heart has ceased to beat (Haller, Nysten). [This beating 

 can also be observed in those veins in a rabbit after the heart is cut 

 out of the body.] 



(2.) The double arrangement of the fibres (transverse and longi- 

 tudinal) produces a simultaneous and uniform diminution of the 

 auricular cavity (such as occurs in most of the hollow viscera). 



(3.) The contraction of the circular muscular fibres around the 

 venous orifices, and the subsequent contraction of the auricle, cause 

 these veins to empty themselves into the auricle ; and by their presence 

 and action they prevent any large quantity of blood from passing back- 

 ward 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 play 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 of the Ventricles. The fibres in the thick 

 wall of the ventricles are arranged in several layers (Fig. 19, 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 trabeculse earner. 

 Between these two layers there lies the thickest layer, consisting of 

 more or less tozs^rse/y-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 surrounded by the primitive bundles 

 of muscular fibres (Henle). 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 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 



