MUSCULAR STRUCTURE AUEICLES. 



317 



THE MUSCULAR FIBRES of the heart in their mode of action belong to the 

 involuntary class, but are of a deep red colour, and possess the transversely 

 striated structure. They are smaller than the ordinary voluntary muscular 

 fibres : their striation is frequently as distinct in a longitudinal as in a 

 transverse direction : and not only is there an exceedingly intricate inter- 

 lacement of both fasciculi and fibres, but the latter appear to divide and 

 unite frequently with each other so as to produce a finely reticulated structure. 



Fig. 236. HEART OP A YOUNQ Fig. 236. 



SUBJECT DISSECTED AFTER 

 BOILING TO SHOW THE SUPER- 

 FICIAL MUSCULAR FIBRES, 



SEEN ANTERIORLY. 



This figure is planned after one 

 of Luschka's, but its details have 

 been hiefly taken from an original 

 preparation. The aorta and pul- 

 monary arteries have been cut 

 short close to the semilunar valves, 

 so as to show the anterior fibres 

 of the auricles. a, superficial 

 layer of the fibres of the right; 

 ventricle ; b, that of the left ; 

 c, c, anterior interrentricular 

 groove, from which the coronary 

 vessels have been removed, show- 

 ing at the upper and lower part 

 most of the fibres passing across 

 from the right to the left ven- 

 tricle, while in the middle part 

 some dip into the septum ; a', 

 pulmonary artery ; &', aorta ; d, 

 right auricle ; d', its appendix, 

 both showing chiefly perpen- 

 dicular fibres; e, upper part of 

 the left auricle ; between e, and 6', 

 the transverse fibres which behind 



the aorta pass across both auricles ; g', appendix of the left auricle ; /, superior vena 

 cava, round which, near the auricle, circular fibres are seen ; g, </, right and left pul- 

 monary veins with circular bands of fibres surrounding them. 



The fibres of the auricles are not continuous with those of the ventricles, 

 the two sets being connected together only by the intervention of the thin 

 fibrous rings round the auriculo- ventricular orifices ; so that when these 

 rings are destroyed by boiling a heart for some hours, the auricles may be 

 easily separated from the ventricles. 



Fibres of the auricles. These consist of a superficial set, common to 

 both cavities, and of deeper fibres proper to each. 1. The superficial common 

 or transverse fibres run transversely over both sinuses, near the base, and 

 are most numerous on the anterior surface : some of them pass into the 

 interauricular septum. The deeper fibres, which are proper to each auricle, 

 consist of two sets, viz. the looped and the annular fibres. 2. The looped 

 fibres pass over the auricle, and seem to be attached by both extremities to 

 the corresponding auriculo- ventricular rings. 3. The annular fibres encircle 

 the auricular appendages from end to end, some longitudinal fibres running 

 within them. These annular fibres also surround the entrances of the venae 

 cavse on the right, and of the coronary vein and the pulmonary veins on the 

 left side of the heart, the muscular fibres extending for some distance 



