THE ARRANGEMENT OF THE MUSCULATURE OF THE HEART 265 



cavity, we observe the orifices of the pulmonary veins, generally four 

 in number. 



The musculature of the principal mass of the auricles is arranged 

 as an outer transverse and an inner longitudinal layer. ^ Moreover, 

 while each auricle really constitutes an anatomical and functional 

 entity, a number of fibers of the superficial coat always pass from one 

 side to the other, thus joining the two. In this way, a coordinated 

 activity of the two chambers is assured. Circular fibers are much 

 in evidence at the orifices of the larger veins and at the coronary sinus. 

 It should be emphasized, however, that these muscular rings do not 

 act as sphincters, but merely tend to lessen the size of the opening. 



A more complicated relationship is presented by the musculature 

 of the ventricles. As these parts are called upon to develop the force 

 necessary to drive the blood through the 

 distant vascular channels, it cannot sur- 

 prise us to find that their walls possess 

 a great massiveness and strength. Fur- 

 thermore, as the left ventricle is destined 

 to supply the blood-vessels of the greater 

 circuit and thus to perform by far the 

 greatest amount of work, it may be as- 

 sumed that its wall is much thicker and 

 stronger than that of the right cavity. 

 In cross-section, the left cavity appears t:. ,or. rr. a 



, , . ^ - 1 1 T Fig. 130. — Transverse Sec- 



as a rounded ormce enveloped by a heavy tion Through Heart of Dog, 3 



frame of muscle-tissue, while the right cm. Above Apex to Show Shape 



compartment presents itself as a cres- ^^^^.^^^^^'^^^ °^ Ventricular 

 cent-shaped slit limited externally as by 



a relatively thin layer of muscle (Fig. 130). It should be remembered, 

 however, that the basal portion of the right cavity gradually assumes 

 a more conical outline, and that the apex of the heart is formed ex- 

 clusively by the left ventricle. Thus, if the heart is divided trans- 

 versely beginning at its apex, the left ventricular cavity is opened 

 first and the right cavity only after another section at a much higher 

 level has been made. 



Although the ventricular muscle fibers do not exhibit definite 

 points of origin and insertion, it is permissible to assume that they 

 begin in the fibrous tissue at the auriculoventricular junction; 

 indeed, the entire ventricular network may be likened to a muscular 

 basket fastened above to the annuli fibrosi. Three distinct layers 

 are discernible, namely, an outer, a middle, and an inner. The fibers 

 of the outer and inner layers are arranged longitudinally, while those 

 of the median coat are directed transversely to the long axis of the 

 heart and pass, therefore, circularly around the lumen of the ventricu- 

 lar cavity. Beginning at the base of the heart, the outer fibers 

 extend spirally toward the apex, but in such a way that their general 



1 Krehl, Abhandl. der sachs. Gesellsch. der Wissenschaften, xvii, 1891, 346. 



