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A TEXTBOOK OF PHYSIOLOGY 



On microscopic examination there may be seen (1) the branched 

 network of cells in the A.-V. node, (2) the large pale cells of the main 

 bundle, (3) the peculiar Purkinje cells constituting the septal divisions 

 and the terminal ramifications. In the human heart the different 

 portions of the bundle are not so easily recognized, but with practice 

 it can be identified both macroscopically and microscopically. It is 

 interesting to note that the invaginated portion of the auricular canal 

 (the A.-V. bundle) has persisted in the part of the primitive cardiac tube 

 which has been least disturbed by the development of the two ventricles. 



There possibly exists another muscular connection between right 

 auricle and right ventricle in the right lateral external part of the 

 A.-V. groove. 



FIG. 31. LEFT VENTRICLE OF CALF. (Keith and Flack.) 



1, Left septal division of A.-V. bundle; 2-3, subaortic musculature divided to show 

 passage of bundle from right side of heart ; 4-5, branches of left septal division 

 passing to 6, moderator bands containing prolongations of bundle to fuse with 

 musculature of heart wall; 9, left auricle; 10, aorta; 11, 12, aortic valves; 

 13, pulmonary artery. 



In the heart of the bird, which contains no sinu-auricular node 

 and no muscular bundle in the position of the A.-V. bundle, a similar 

 muscular connection has been found in the posterior aspect of the 

 A.-V. groove in the region of the left superior vena cava. 



Microscopic Anatomy. The vertebrate heart-muscle consists of 

 fibres, which in their turn are composed of fibrils, or sarcostyles and 

 sarcoplasm. Nuclei are situated at regular intervals in the fibres, 

 and are surrounded by a small mass of granular protoplasm the 

 sarcoplasm. Running out from the central sarcoplasm to the periphery 

 between the fibrils there is a very delicate protoplasmic membrane 



