The superior vena cava and adjacent right atrium containing the sinoatrial node in each of five human hearts was studied histologically in serial scction. The tissue block of a 41 year old man was reconstructed in four colors to provide a three-dimensional model of the sinoatrial node, atrial relations, blood supply and nodal configuration. The resulting model demonstrated the curved course of the compact sinoatrial node and the mural relations of its tapered superior and inferior ends.Microscopic measurements of the boundaries of the five human nodes yielded an epicardial to endocardial mean thickness in the compact body of the node of 1.6 mm, that of the lower node was 0.6 mm. The mean length of the five nodes was 7.3 mm. Such microscopic delineation of the node is more accurate than gross measurements and indicates that the size of this vital mass of pacemaker tissue in man is smaller than the larger meafiurements usually given in the literature. Small strands of nodal muscle fibers follow longer or shorter courses before they become continuous with the larger more darkly stained atrial cardiac muscle fibers. We found no histologic evidence within the human node, or along its periphery, of continuity between the small nodal fibers and very large atrial fibers.

The right atrioventricular junction and A-V node area in each of five human hearts was studied histologically in serial sections. The A-V node of a 54-year old female was reconstructed in four colors to provide a three-dimensional model of cardiac relationships, blood supply, and nodal configuration. The nodal fibers demonstrated two laminations. A superficial layer was composed of longitudinally oriented fibers, whereas a deeper layer was composed of oblique and transversely directed fibers. The atrioventricular bundle fibers were always continuous with nodal fibers of the deeper portion of the A-V node. Atrionodal fiber junctions occur along the superior, endocardia1 and inferior borders of the A-V node and impart to these surfaces a spiked appearance. The potential significance of A-V node structure to atrial cardiac conduction is presented.

The gross distribution, histologic structure, blood supply, innervation, and physiologic implications of this unique fiber system has continued to attract the research interests of investigators from many disciplines of science. The resulting body of diverse information is a voluminous and indeed a fascinating scientific story.A complete survey of even the essential achievements is beyond the limited time and scope of this presentation. However, the interested reader can obtain an appreciation of advancements through a perusal of several morphologic studies that include extensive citations from the early and current literature.' ' The comparative histology of the cardiac conduction system in reptiles, birds, and mammals has also been presented and reviewed.' '" T h e macroscopic relations a n d distribution of t h e conduction system in man and other mammals have been revealed by high magnification dissection," l 3 three-dimensional reconstructions," l6 histochemical ~t a i n i n g , '~ and painting the endocardia1 surface with dyes." ' " For detailed studies of the SA and AV nodal areas of man and other mammals the reader is referred to the contributions of Widran and Lev," James,I7 ") Titus et al.," and Truex et ~1 .~ The ultrastructure of the same nodal areas also has been reported.6.21." I n recent years we have obtained and studied the hearts of several species of animals in an attempt to resolve some of the questions pertaining to the presence or absence of the various portions of the conduction system in different animals. By the use of several methods (e.g., injection, microdissection, radiographic visualization, serial histologic sections, and model reconstructions), we have examined parts, or all, of the cardiac conduction system elements in the animals shown on the right side of F I G U R E 1. Differential stain procedures included hematoxylin and eosin, variations of the Masson's trichrome, azophloxine, Bodian or Holme's silver methods. These comparative studies are, as yet, incomplete and many additional animals in each of the groups remain to be examined. The purposes of this brief communication are to summate only our current information on the morphology of the several conduction elements, and to emphasize several structural characteristics which may enhance the interpretation of the physiological papers which appear in this monograph. We present these preliminary anatomic observations with the full realization that they may,

This nucleus was identified in four cats, four dogs and nine human spinal cords, but was not identifiable in seven other human specimens. In all three species the nucleus extends rostrally into the lower third of the medulla. Cell measurements of the perikarya yielded a mean diameter of 34.1 p in the dog, 26.7 / . L in the cat, and 15.7 it in man, Nuclear diameters as well as the nucleus-cytoplasm and volume ratios were determined. The lateral cervical nucleus is considered to be rudimentary in man and was a well-defined structure in only 2 of 16 specimens. In one of these two cases a total of 5173 cells were counted in the nuclei of both sides at the C 1 arid medullary levels. Thc pertinence of these observations to the preservation of tactile sensation after injury of the dorsal columns in man, is discussed.The presence and location of a lateral cervical nucleus (LCN) in the cat has been established in a number of anatomical studies (Rexed and Brodal. '51: Rexed and Stram, '52: Brodal and Rexed, '53; Morin, '55; Morin and Catalano, '55; Rexed, '54; '64; Ha and Liu, '66; Westman, '68a,b; Grant and Westman, '69). This small group of neurons also has been identified in the dog (Kitai, Ha and Morin, '65). A lateral cervical nucleus was described in the monkey by Gardner and Morin ('57), Ha and Morin ('64), and Mizuno et al. ('67).Rexed ('58) stated that this nucleus was present in the dog, sheep, seal and whale but non-existent in the cervical cord of the rat, guinea pig, rabbit and possibly man. Gardner and Morin ('57) examined four human cervical cords and observed no lateral cervical nucleus. Similar negative findings were reported by Rexed ('58) and Seki ('62, '65 MATERIALS AND METHODSThe location of the neurons that cornprise the LCN of the cat and dog are well established. Transverse and longitudinal sections of these two species ( 4 cats and 4 dogs) were used for purposes of comparison and normal controls. Paraffin or frozcn sections of the cervical cord ( C l -3 ) and the lower half of the medulla were stained by either the cresyl violet, Lux01 blue-cresyl violet, Holmes silver-Luxol blue, or thionin technics.The human material used in the present study consisted of cervical cord and medulla segments from one newborn and 15 adults (38 to 76 years of age). Paraffin or frozen sections cut in two planes were stained by one or more of the methods noted above. Two each of the cat, dog and human specimens were sectioned and 9394 TBUEX, TAYLOR, SMYTHE AND GILDENBERG stained serially. The latter material was used to provide cell measurements of 100 neurons in each of the six specimens. All micrometer measurements ( fig. 1 ) were made with an oil immersion lens at a magnification of X 970, using the method described previously by Truex, Scott, Long and Smythe ('55). RESULTSA well developed lateral cervical nucleus (LCN) was present in all eight specimens of the cat and dog cervical spinal cord ( fig. 3 ) . A well defined LCN was observed in two human adult specimens. but the nucleus was poorly develo...

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