90 MUSCULATURE OF THE VENTRICLES. 



tend upon the pulmonary veins as far as the hilus of the lung; in other animals 

 (apes, rats) they extend even into the lung itself; indeed, in some mammals 

 (mouse, bat) this muscular layer penetrates the lung so far that in the small 

 veins the entire wall is composed almost wholly of striated muscle-fibers. Muscle- 

 fibers, chiefly circular, are also found at the termination of the great cardiac 

 vein and in the coronary valve of Thebesius. Many elastic fibers are present 

 in the perimysium of the auricles. 



From the physiological standpoint the foregoing anatomical data 

 explain the following facts with relation to the contractions of the 

 auricles. 



The auricles are able to contract independently of the ventricle's; 

 this is particularly manifest in the cessation of the heart's activity, as 

 under such circumstances two or more contractions of the auricle alone 

 are often seen to take place, followed now and then by a single con- 

 traction of the ventricle. However, when the action of the heart is 



n 



FIG. 22. I, Course of the Muscle-fibers in the Left Auricle: the outer transverse and the inner longitudinal fibrous 

 layer are visible and in addition the circular fibers of the pulmonary veins, v.p. V, left ventricle (Joh. Reid). 

 II, Distribution of Transversely Striated Muscle-fibers on the Superior Vena Cava (Elischer): a, entrance 

 of the azygos vein; v, auricle. 



unimpaired the auricles in their contraction transmit the motor impulse 

 to the ventricles. Whether this stimulation is brought about through 

 nerve-fibers or, as is more probable, through connecting muscle-bundles, 

 has not yet been decided with certainty. 



The two chief layers of fibers (transverse and longitudinal), which 

 cross each other, serve to effect uniform contraction of the auricular 

 cavity from all sides, as is the case likewise with most hollow muscular 

 organs. 



The circular fibers surrounding the entering venous trunks, through 

 their contraction, which occurs in unison with that of the auricles, 

 cause in part an emptying of blood into the auricle and in part a hin- 

 drance to a return of the blood in any considerable measure. 



ARRANGEMENT OF THE MUSCULATURE OF THE VENTRICLES. 



The Muscle-fibers of the Ventricles. Beneath the pericardium there is first 

 met an outer longitudinal layer (Fig. 23, A), consisting of only occasional 

 bundles on the right ventricle, while on the left it comprises a compact layer of 

 about one-eighth of the entire thickness of the wall. A second layer of longitu- 

 dinal fibers lies on the inner surface of the ventricles, being especially well marked 

 at the orifices, as well as inside the perpendicularly placed papillary muscles, 



