502 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



on the left ventricle, constituting about one-eighth of the thickness of the 

 ventricular wall (Fig. 196). " 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, while elsewhere it is 

 replaced by the irregularly arranged trabeculae carnse. Between these 

 two layers there lies the thickest la}^er, consisting of more or less trans- 

 versely arranged bundles, which may be broken up into single layers 

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

 the layers, while 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 arid 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 longi- 

 tudinal layers (as is shown schematically in 0, Fig. 196), 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 (Wirbel), B, and where 

 they enter the muscular substances, and, taking an upward and inward 

 direction, reach the papillary muscles (Lower), D, although it is a 

 mistake to say that all the bundles which ascend to the papillary muscles 

 rise from the vertical fibres of the outer surface ; many seem to rise inde- 

 pendently within the ventricular walls. 



"According to Henle, all the external longitudinal fibres do not rise 

 from the fibrous rings, but the roots of the arteries." 



The Movements^/ the Heart. The movements of the heart can be 

 most readily studied in their simplest form in the frog. 



The batrachian heart, as seen in Fig.. 187, is an egg-shaped mass slightly flat- 

 tened at the sides and marked by a furrow which crosses the heart nearly at right 

 angles to its axis, dividing the heart (in an anterior view) into an upper globular 

 part, the two auricles, and a lower conical, the single ventricle ; the anterior sur- 

 face of the ventricle is also marked by a slight groove inclining from above down- 

 ward toward the right of the heart. Anteriorly, the ventricle is seen to be con- 

 tinuous with a cylindrical prominence, the bulbus arteriosus, which crosses over 

 the right auricle and divides into the right and left aortse. In viewing the pos- 

 terior part, the right auricle is seen to be continuous with a bulbar expansion of 

 the inferior vena cava, which receives the name of the sinus venosus, the line of 

 junction of the sinus with the auricle being marked by a slight furrow. The two 

 auricles are separated from each other by an antero-posterior septum, incomplete 

 at its lower margin, where the two auricles communicate through a crescentic 

 opening ; the opening of the sinus into the auricle is marked by an Eustachian 

 valve, which hangs downward and toward the right. 



The auriculo- ventricular valve consists of an anterior and posterior segment, 

 each of which is continuous at its edge with the inter-auricular septum. 



