638 SPECIAL PHYSIOLOGY. 



conjoined at one point. Behind the aortic orifice, and between the two 

 auriculo-ventricular openings, is a piece of fibro-cartilage, which, when 

 ossified, forms the bone of the heart. 



The heart, though an involuntary muscle, has dark red fibres, marked 

 with transverse striae, like those of the voluntary muscles; but they 

 differ, in being often branched and joined together again, and in having 

 their fasciculi interlaced ; moreover, the fibres are somewhat smaller, 

 less distinctly striated, sometimes marked by faint longitudinal streaks, 

 and, for the most part, are not attached to tendinous structures: the 

 sarcolernma of each fibre is not easily seen, excepting in fatty degen- 

 eration of the fibres. There is scarcely any areolar tissue between the 

 fibres, hence the characteristic firmness of the contracted heart. 



The arrangement of the muscular fibres of the heart is one of the 

 most difficult subjects of investigation. Vesalius, Albinus, and even 

 Haller, were unable to follow them ; and very different and complicated 

 descriptions of the fibres have been given by Lower, Winslow, Senac, 

 Wolff, Gerdy, Duncan, Reid, Budge, and, more recently, by Pettigrew. 



After long boiling or maceration in pure alcohol, the fibres of the 

 auricles, but especially those of the ventricles, may be stripped off, 

 and unwound in flat bands or layers. The auricular and ventricular 

 fibres are quite independent, the auriculo-ventricular fibrous rings, 

 with the pericardium and endocardium, forming the sole bond of union 

 between them. 



The fibres of the auricles consist of a superficial and deep layer. 

 The superficial layer is thin, incomplete, and common to both auricles ; 

 its fibres are transverse, and chiefly found on the front of the auricles ; 

 a few enter the septum. The deep layer consists of looped and an- 

 nular fibres proper to each auricle ; the looped fibres are fixed, by both 

 ends, to the corresponding fibrous ring, and pass, in various directions, 

 over the surface of the auricle ; the annular fibres embrace the auric- 

 ular appendages, and also surround the large venous openings in each , 

 auricle, extending both over the auricle and the veins. 



The fibres of the ventricles, first studied by Lower (1669), present 

 a far more complex arrangement. They present numerous, and some- 

 what easily separable, layers, having a general spiral arrangement 

 around those cavities. The superficial fibres pass, more or less ob- 

 liquely, downwards from right to left, on the front of the ventricles, 

 and, in the opposite direction, i. e., upwards from right to left, at the 

 back of the ventricles. A few of these fibres are almost vertical. On 

 the posterior or under surface of the heart, many of the superficial 

 fibres are common to both ventricles, passing from one to the other, 

 over the line of the septum, the surface being flattened, and the two 

 ventricles blended together. On the upper surface, or in front, 

 however, a great number of the superficial fibres dip in at the line of 

 the septum, decussating with each other, so that the ventricles are here 

 more distinct ; nevertheless, some of these fibres pass from one ven- 

 tricle to the other. When these are removed, the deeper fibres are 

 found to be chiefly proper to each ventricle ; so that, as said by Wins- 

 low, the ventricular portion of the heart seems to be composed of two 

 hearts, enveloped in a third. As the fibres become deeper, they are 



