THE HEART. 



each other ; both, however, have as their chief point of origin the ostia 

 venosa of the ventricles, where tough, tendinous tracts the so-called 

 annuli fibro-cartilaginei are situated, two anterior, on the right and 

 left of the aortal opening, and one posterior, which runs backwards 

 also from the aorta to the border of the auriculo-ventricular septum, 

 where it splits into two slender crura. In the auricles are found : 

 1, fibres, which are common to both, in the form of transverse, flattened 

 bundles, which proceed chiefly anteriorly, but afterwards also superiorly 

 and posteriorly, from one auricle to the other, and are continued in 

 them as transverse fibres. 2, special fibres. These constitute, in the 

 first place, complete rings at the origins of the great veins, and at the 

 points of the appendices ; and, in the second place, a longitudinal layer 

 of some thickness beneath the endocardium, which springs from the 

 auriculo-ventricular openings, and is especially developed in the right 

 auricle (musculi pectinati). Besides these, there exist between the latter 

 muscles, and also in the auricles, numerous other small fasciculi, which, 

 on account of their irregularity, cannot be more particularly described. 

 The septum is to some extent common to both auricles. Its muscles 

 arise from the most anterior part of the upper border of the septum of 

 the ventricles, immediately behind the aorta, from the posterior fibro- 

 cartilage and arch to the right, around the fossa ovalis, in which only 

 slender fibres exist, in a superior and posterior direction, in order to 

 terminate, partly at the vena cava inferior, partly by forming a com- 

 plete ring ; whilst on the left, they surround the fossa ovalis in the 

 opposite direction. 



The muscular structure of the ventricles is disposed so that on the 

 external and internal surfaces the fibres everywhere decussate ; and in the 

 intermediate portion, every stage of transition from the one direction to 

 the other is presented. The muscular fibres arise at the ostia venosa 

 and at the arterial openings, in part immediately, and partly with the 

 intervention of short tendons, run more or less obliquely (and some 

 longitudinally or even transversely), and after they have surrounded a 

 portion of the ventricle in the longitudinal or transverse direction, curve 

 back again, and then terminate, some in the musculi papillares, and 

 chordce tendinece, whilst others are again inserted in the points of origin 

 above indicated, so that without being interrupted by tendons, they 

 describe large involved loops, or figure-of-8 turns of large size, and 

 running in very numerous and diverse directions. 



The endocardium is a whitish membrane, investing all the elevations 

 and depressions of the internal surface of the heart, as well as the papil- 

 lary muscles and their tendons, and the valves. It is most developed in 

 the left auricle (as much as J of a line), and thinnest in the ventricles, 

 so that the muscular substance there presents its natural color. As 

 regards its structure, it consists of three layers ; an epithelium, an 



