ARKANGEMENT OF THE MUSCULAR FIBRES. 



257 



some of them pass in at the inter-auricular septum (fig. 175). The 

 deeper fibres, which are proper to each auricle, consist of two sets, viz., 

 the looped and the annular fibres. The looped fibres pass over the 

 auricle, and seem to be attached by both extremities to the correspond- 

 ing auriculo-ventricular rings. The annular fibres encircle the 

 auricular appendages (fig. 175, d', 176, e'), some longitudinal fibres 

 running within them. These annular fibres also surround the entrances 

 of the venas cava (/, /) on the right, and of the coronary vein and the 

 pulmonary veins on the left side of the heart {g, li), — the muscular 

 fibres extending for some distance from the auricle upon the vems, 

 especially upon the superior vena cava and the pulmonary veins. 



Fibres of the ventricles. — The muscular fasciculi of the ventricles 

 have a very intricate disposition, which has received great attention 

 from anatomists. Many of the statements, however, are conflicting, 

 and it must be confessed that the subject still admits of further 

 investigation. 



Fig. 176. — Posterior view of 



THE SAME PREPARATION AS 

 IS REPRESENTED IN THE PKE- 



CEDiNii FIGURE (Allen Thom- 

 son). 



a, posterioi' surface of the 

 right ventricle with its super- 

 ficial muscular fibres dissected ; 



b, the same of the left ventricle ; 



c, posterior interventricular 

 groove, from which the coronary 

 vessels have been removed ; d, 

 right auricle ; e, the left, show- 

 ing some transverse fibres common 

 to both auricles, and others be- 

 longing to each ; /, superior vena 

 cava ; (/, <j' laulmonary veins cut 

 short ; h, sinus of the great 

 coronary vein covered by muscular 

 fibres ; W posterior coronary vein 

 joining the principal one ; i, in- 

 ferior vena cava ; i! Eustachian 

 valve. 



It is chiefly the constant 

 twisting and overlapping 

 of the several bundles 

 which renders difficult the 

 investigation of their course 

 and disposition. In order to unravel them with any degree of success, it 

 is best to boil the slightly distended heart for a short time, so as to 

 soften the connective tissue, and then carefully to dissect the organ in 

 part by cutting and in part by tearing asunder the fibres with blunt 

 instruments.* 



The surface fibres of the ventricles (figs. 175, 176, 177) extend from 

 the base, where they are attached to the tendinous structures around 



* For convenience of description, the heart, in the following account of the course of 

 the fibres, is supposed placed apex downiwards, and with the anterior and posterior sur- 

 faces about equally occupied by the two ventricles (as represented in figs. 175, 176). 

 VOL. II. s 



