ARRANGEMENT OF THE CARDIAC MUSCULAR FIBRES. 67 



the fibres. The fibres are gathered lengthwise in bundles, or fasciculi, 

 surrounded and separated from each other by delicate processes of the 

 perimysium. When the connective tissue is dissolved by prolonged 

 boiling, these bundles can be isolated, and constitute the so-called 

 " fibres " of the heart. The transverse sections of the bundles in 

 the auricles are polygonal or rounded, while in the ventricles they 

 are somewhat flattened. [The muscular mass of the heart is called the 

 myocardium, and is invested by fibrous tissue. It is important to 

 notice that the connective tissue of the visceral pericardium (epicardiuvi) 

 is continuous with that of the endocardium by means of the peri- 

 mysium surrounding the bundles of muscular fibres.] The fine spaces 

 which exist between these bundles form narrow lacunse, lined with 

 epithelium, and constituting part of the lymphatic system of the heart. 



[The cardiac muscular fibres occupy an intermediate position between striped 

 and plain muscular fibres. Although they are striped they are invohintary, not 

 being directly under the influence of the will, while they contract more slowly 

 than a voluntary muscle of the skeleton.] 



[In the/ror/'s heart the muscular fibres are in shape elongated spindles, or fusi- 

 form, in this respect resembling the plain muscle-cells, but they are transversely 

 striped (Fig. 17, C). They are easily isolated by means of a 33 per cent, solution of 

 potash or dilute alcohol (Weissmann, Ranvier).] 



44. Arrangement of the Cardiac Muscular Fibres, 

 and their Physiological Importance. 



The study of the embryonic heart is the key to a proper understand- 

 ing of the complicated arrangement of the fibres in the adult heart. 

 The simple tubular heart of the embryo has an outer circular and an 

 inner longitudinal layer of fibres. The septum is formed later ; hence, 

 it is clear that a part, at least, of the fibres must be common to the 

 two auricles, and a part also to the two ventricles, since there is, 

 originally, but one chamber in the heart. The muscular fibres of the 

 auricles are, however, completely separated from those of the ventricles 

 by the fibro-cartilaginous rings. In the auricles the fundamental 

 arrangement of the embryonic fibres partly remains, while in the 

 ventricles it becomes obscured as these cavities undergo a sac-like 

 dilatation, and also become twisted in a spiral manner. 



(1.) The Muscular Fibres of the Auricles are completely separated 

 from the fibres of the ventricles by the fibrous rings which surround 

 the auriculo-ventricular orifices, and which serve as an attachment for 

 the auriculo-ventricular valves (Fig. 18, I). The auricles are much 

 thinner than the ventricles, and their fibres are generally arranged in 

 two layers ; the outer transverse layer is continuous over both auricles, 



