404 MANUAL OF HISTOLOGY. 



muscle, collected into bundles, excepting the trabeculce. carneoe, in.pectinati 

 and papillares. The single fibres lie rather closely crowded side by side, 

 held together by a small quantity of connective-tissue. 



As is well known, the strength of the fleshy mass varies much in the 

 different divisions of the heart. It is most massive in the left ventricle, 

 thin in the two auricles, and weakest in the right of these. The course 

 which the fibres take is also very complicated, for which reason we shall 

 confine ourselves to only a few of the chief points of interest as regards it. 



The course of the fibres of the heart, which is different in the auricles 

 and ventricles, may be divided into longitudinal and circular. This 

 distinction, however, can only be made with accuracy as regards the 

 auricles, and not the ventricles. It is a remarkable fact, further, that 

 some of the muscular fibres are common to the two auricles, and another 

 to the two ventricles, while each of these four parts possesses also its 

 special fibres. 



The starting-points of the fibres of the heart are usually held to be 

 the two annular masses of fibres which encircle the ostia venosa of the 

 ventricles, known as the annuli fibro-cartilaginei. They consist of very 

 strong connective-tissue, with very delicate elastic fibres. Sometimes 

 their tissue assumes a similar appearance to that of the perichondrium at 

 its transition into true cartilaginous tissue. 



From these rings the fibres take their origin, and return, after travelling 

 round the cavities of the organ, to be inserted into them again, thus 

 forming loops. In consequence of this, both auricles and ventricles must 

 contract towards these points, the Bases of the ventricles, during systole 

 of the organ. 



In the auricles we encounter in the first place, as most internal layer, 

 bundles of fibres springing from tbe ostium venosum, and forming a series 

 of loops, which arch over the cavity, producing a kind of dome. From 

 their peculiar development in the right auricle they give rise to the m. 

 pectinati. This layer is enveloped by another stronger one, formed of 

 circular fibres, which is in the first place distinct for each of the auricles, 

 and then specially developed on the anterior aspect of the organ, it 

 includes both of them in common. Finally, surrounding the openings 

 of the veins we find circular fibres, continued to a certain distance over 

 the wafts of these vessels. 



The arrangement of the fibres of the ventricles, however, is more 

 complex. In the first place, it may be remarked that the left ventricle 

 possesses a special set of fibres. The right has likewise its own, which 

 are, however, so arranged as to strengthen the muscular mass of the left, 

 being produced into it. Finally, fleshy fibres are to be seen, which, 

 starting from the left ventricle and returning to the same, surround in 

 their course the right cavity in loops. 



It may be remarked, namely, that from the fibrous ring of the left side, 

 and from the aorta also, in the whole circumference of the ventricle, a 

 number of longitudinal fleshy fibres take their origin, which descend on 

 the one wall in its outer portion, and bending round at the apex of the 

 heart, return in the inner surface of the opposite wall to the annulus fibro- 

 cartilagineus. Owing to the oblique course of these fibres, they cross 

 each other at the apex of the left ventricle, forming there the so-called 

 vortex of the heart. In the right ventricle, likewise, we meet with an 

 origin of fibres from the annulus fibro-cartilagineus. There one limb of the 

 loop pursues a course in a similar manner down to the apex of the right 



