PERICARDIUM; ENDOCARDIUM; VALVES. 91 



while in other situations it is replaced by the irregularly running fibers of the 

 muscular trabeculae. Between the two longitudinal layers lies the most powerful 

 transverse layer, the fibers of which are separable into individual, leaf -like, 

 ring-shaped bundles. The three layers, however, are not wholly independent and 

 separated from each other, but rather there is a gradual transition between the 

 transverse and the outer and inner longitudinal layers by means of oblique fibers. 

 The common assumption is that the entire outer longitudinal layer passes 

 gradually into the transverse and this in turn wholly into the inner longitudinal, 

 as is shown diagrammatically in Fig. 23,0. This is not justifiable, and is negatived 

 by the great preponderance in the thickness of the middle layer. In general the 

 outer longitudinal fibers pursue such a course as to intersect the course of the 

 fibers of the inner longitudinal layer at an acute angle. The intervening trans- 

 verse layer constitutes the medium for a gradual transition between these courses. 



FIG. 23. Course of the Muscle-fibers in the Ventricles: A, course upon the anterior surface; B, view of the apex 

 with the "whirl" (Henle); C, diagrammatic representation of the course of a muscle-fiber within the wall 

 of the ventricle: D, course of such a fiber into the papillary muscle (C. Ludwig). 



At the apex of the left ventricle external longitudinal fibers, uniting in the 

 so-called "whirl" (B), pass in a curved direction inward and upward within the 

 muscle-substance and extend into the papillary muscles (D). Nevertheless it is 

 an error to consider that all of the ascending fibers in the papillary muscles are 

 derived from these vertical muscle-bundles of the outer surface, as many arise 

 independently from the wall of the ventricle. Neither can the origin of these 

 longitudinal fibers on the outer surface of the heart be traced solely to the fibrous 

 rings or to the roots of the arteries. Finally, mention should be made of the special 

 circular layer of fibers that surrounds the left orifice like a sphincter. Numerous 

 lymph-vessels are present in all the interstices between the muscle-fibers and the 

 blood-vessels. These eventually empty into the lymph- vessels and nodes of 

 the mediastinum. 



PERICARDIUM ; ENDOCARDIUM ; VALVES. 



The pericardium, which includes between its two layers a lymph-space the 

 pericardial cavity containing a small amount of lymph, exhibits the structure of 

 a serous membrane; that is, it is composed of connective tissue containing delicate 



