STRUCTURE AND MUSCULAR FIBRES OF THE HEART. 



51 



capsules, and the frog in its kidneys. When an artery splits up into fine branches during its 

 course, and these branches do not form capillaries, but reunite into an arterial trunk, a rete 

 mirahile is formed, such as occurs in apes and the edentata. Microscopic retia mirabilia exist 

 in the human mesentery (Schobl). Similar arrangements may exist in connection with veins, 

 giving rise to venous retia mirabilia. 



43. THE HEART. The muscular fibres of the mammalian heart consist of short (50 to 

 70 [j. in man), very fine, transversely striated fibres, which are actual unicellular elements, devoid 

 of a sarcolemma (15 to 25 jx broad), and usually divided at their blunt ends, by which means they 

 anastomose and form a network (fig. 26, A, B). The individual muscle-cells contain in their 



Fig. 26. 



A, muscular fibres from the heart of a mammal, and C from a frog ; B, transverse section of the 

 cardiac fibres ; b, connective-tissue corpuscles ; c, capillaries. 



centre an oval nucleus, and are held together by a cement which is blackened by silver nitrate, 

 and dissolved by a 33 per cent, solution of caustic potash. This cement is also dissolved 

 by a 40 per cent, solution of nitric acid. The transverse strise are not very distinct, 

 and not unfrequently there is an appearance of longitudinal striation, produced by a 

 number of very small granules arranged in rows within 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 (epicardium) is continuous with that of the endocardium by 

 means of the perimysium surrounding the bundles of muscular fibres.] The fine spaces which 

 exist between these bundles form narrow lacuna?, 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 involuntary, not being directly under the 

 influence of the will, while they contract more slowly than a voluntary muscle of the skeleton.] 

 In the frog's heart the muscular fibres are in shape elongated spindles, or fusiform, in this 

 respect resembling the plain muscle-cells, but they are transversely striped (fig. 26, C). They 

 are easily isolated by means of a 33 per cent, solution of potash or dilute alcohol. 



44. ARRANGEMENT OF THE CARDIAC MUSCULAR FIBRES. The 



study of the embryonic heart is the key to a proper understanding 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 the cavities undergo a sac- 

 like dilatation, and also become twisted in a spiral manner. 



