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A TEXTBOOK OF PHYSIOLOGY 



and from the left posterior the left coronary artery. These vessels 

 supply the heart muscle. 



Various accounts are given of the arrangement of the musculature 

 of the ventricles. Internally, the muscular fibres are thrown into 

 columns the columnse carnae and the papillary muscles. The super- 

 ficial fibres take origin from the auriculo-ventricular ring, and wind 

 spirally about the heart, to end in the papillary muscles, or pass up 

 in the septum to the ring again on the inner surface of the heart. 

 The middle layers, which form the bulk of the tissue, consist of bundles 

 of fibres running more or less circularly round the ventricles. 



FIG. 27. GENERALIZED TYPE OF VERTEBRATE HEART. (Keith.) 



o, Sinus venosus and veins; b, auricular canal; c, auricle; d, ventricle; e, bulbus cordis; 

 /, aorta; 1-1, sinu-auricular junction and venous valves; 2-2, canalo-auricular 

 junction; 3-3, auricular part of auricle; 4-4, invaginated part of auricle; 

 5, bulbo-ventricular junction. 



By the study of the primitive type of vertebrate heart a clear 

 concept has been gained not only of anatomical arrangement, but of 

 the function of certain parts in the more highly developed mam- 

 malian heart. 



The heart develops as a tube (Fig. 26), and the auricle is regarded 

 as a dorsal expansion of this tube, and the ventricle as a ventral 

 expansion. 



The diagram (Fig. 27) represents the general type of a primitive 

 vertebrate heart. The cardiac tube begins at (a) and ends at (/). 

 Dorsally is placed the expansion (c), the auricle, while (d) represents 

 the ventricular outgrowth. Such a heart may be said to consist of five 

 chambers. At the venous end (a) the sinus venosus is formed by 



