290 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



FIG. 4. Purkinje distribution near the 

 endocardium in the goat. The dark 

 muscle fibers below are ordinary myo- 

 cardium; the lighter ones above are the 

 specialized cells which line the endo- 

 cardium. Oriented toward the bottom 

 and to the right is a small strand of 

 Purkinje tissue penetrating the muscle. 

 Note the gray circles and tubes around 

 the laige subendocardial layer. These 

 are portions of the sheath which sur- 

 rounds many of the specialized cells. 

 No sheath surrounds the small fibers 

 which penetrate the muscle. [From 

 Aagaard & Hall (1).] 



bundle terminate near the anterior and posterior 

 papillary muscles. The bundles give rise to numerous 

 Purkinje fibers subendocardially on the right and left. 



Many of the finer rainifications of this Purkinje 

 network can be seen as small strands of muscle crossing 

 the cavities between the trabeculae, particularly on 

 the left and in the apical region of the heart. The 

 specialized tissue is more clearly identifiable, both 

 grossly and histologically, in the ungulates. Portions 

 of it can, however, be grossly identified in the dog 

 and in man. For instance, the right bundle is often 

 readily apparent to the moderately trained eye, as 

 are the branches of the left bundle. The fine fibers 

 which cross the cavities are also clearly identifiable. 

 In the ungulates, the larger branches of the Purkinje 

 system are encased in a connective tissue sheath which 

 can be injected with dyes to delineate a large part of 

 the course of the Purkinje fibers. It has, however, been 

 shown (i) that many of the fine Purkinje branches 

 which penetrate the muscle in the ungulates are 

 not surrounded by this sheath (fig. 4). It is un- 

 doubtedly necessary that the sheath disappear before 

 Purkinje fibers can excite the myocardium. The 

 characteristics of Purkinje fibers in the birds and 

 various mammalian species have been summarized 

 by Truex & Copenhaver (135) with particular 

 reference to the glycogen content. 



"In comparison with cardiac muscle fibers, 

 Purkinje fibers are usually larger, contain fewer 

 myofibrillae, have more interfibrillar sarcoplasm, and 

 give a distinctly clearer less compact appearance. The 

 center of the fiber, being devoid of myofibrillae, is 

 filled with a granular sarcoplasm in which the nuclei 

 are usually in groups of two or more. . . until recently 

 it was assumed that the Purkinje fibers of the conduc- 

 tion system were comparatively rich in glycogen, 

 whereas the cardiac mu.scle had only a meager amount 

 or no glycogen." 



FIG. 5. Normal lead II electrocardiogram. Initial, low, 

 rounded deflection about 1 mm high and 2 mm long is the 

 P wave. Second deflection, about [o mm high and i mm wide 

 shows a rapid rise and fall and is the QRS complex. Third, 

 peaked deflection, about 3.5 mm high and 6 mm long, is the 

 T wave. Sequence is repeated three times. Standardization at 

 right, I mv. Small black vertical lines are 40 msec apart; 

 larger lines (five spaces) are 200 msec apart; heaviest lines are 

 I sec apart. [From VVinsor (146).] 



They go on to give data concerning relative 

 amounts of glycogen in myocardium and Purkinje 

 tissue. In their own studies Purkinje fibers were 

 found to be about twice the size of ventricular myo- 

 cardial fibers in the sheep (30.9;^ vs. 12.6^), pig 

 (37.2/.! vs. i4.3;u), calf (26.5/i vs. ii.i/z) and beef 

 (43.6/i vs. 15.011), whereas in man the relation was 

 closer to I : I (20.6/n vs. i6.i/i). The relations in cat 

 and monkey were similar to that in man. Truex and 

 Copenhaver further comment on the frequency of 

 Purkinje fibers in the moderator band in various 

 species. The glycogen content was quite variable in 

 their specimens. It is worthwhile in passing to note 

 that the existence of a conduction system in the dog 

 and in man was quite recenth' denied by some 

 anatomists (47) who had at one time a considerable 

 following. 



Accessory Pallnvny for A-]' Conduction 



In Kent's studies (64) he stated that the A-\' node 

 and its Purkinje branches are not the sole connection 

 between the atria and \entricle. He found an apparent 



