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A TEXTBOOK OF PHYSIOLOGY 



a grain of wheat. It has a special blood-supply, and here the nerves, 

 of the heart form an intimate contact with the musculature. Micro- 

 scopically, it consists of pale cardiac muscle fit res, with which the 

 nerve fibres appear to become actually continuous. This tissue, 

 apparently intermediate in nature between muscle and nerve, is 

 characteristic of the sinu-auricular and auriculo-ventricular nodes, 

 and is termed " nodal tissue." The nodal tissue becomes less in 

 amount and more concentrated in position as one passes from the lower 

 to the higher type of heart. With the increasing specialization of 

 the organ the extra-cardiac nerves enter into a more direct relation- 

 ship with the cardiac musculature ; the heart to a certain extent loses 

 its independence, and becomes more a part of the general organization. 



FIG. 29. A, SINU- AURICULAR JUNCTION IN HUMAN HEART; B, SINU-AURICULAR. 

 JUNCTION IN TURTLE'S HEART. THE FIGURES REPRESENT CORRESPONDING 

 PARTS IN THE Two HEARTS. (Keith and Flack.) 



1, Musculature of superior vena cava or sinus in A, of sinus venosus in B ; 

 2, artery and surrounding nodal tissue at sinu-auricular junction; 3, position of 

 venous valve in A. In B, 3 indicates junction of musculature of sinus and 

 auricle in the venous valve; 4, auricular muscle, differs from sinus musculature 

 in both A and B in having a very slight endocardial covering; 5, subepicardial 

 tissue; 6, connective tissue between sinus and auricle. 



The auricular canal has also become profoundly modified, owing 

 to the development of two auricles instead of one. The basal part 

 is represented chiefly by the interauricular septi m. 



Similarly, the development of two ventricles by the downgrowth 

 of an interventricular septum has brought about a rearrangement of 

 the auricular ring and the invaginated portion of the auricular canal. 

 Most of the auricular ring has become indistinguishable from the rest 

 of the auricular tissue, but at the base of the interauricular sept 1 m 

 there is found a mass of " nodal " tissue, known as the auriculo- 

 ventricular or A.-V. node (3, Fig. 30). 



