The law of cardiac muscle with special reference to conduction in the mammalian heart / Thomas Lewis.

  • Thomas Lewis
Date:
[1921]
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Licence: Public Domain Mark

Credit: The law of cardiac muscle with special reference to conduction in the mammalian heart / Thomas Lewis. Source: Wellcome Collection.

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    specialized structures which form in the mammal the union between the auricular and ventricular portions of the heart. Here it traverses the auriculo- ventricular node; this node is composed of very small interlacing fibres which, like those of the s.-a. node, are richly supplied by nerves. Next it passes through the bundle and its branches; it traverses these and thus arrives simultaneously at many points of the endocardial surface of the right and left ventricles. The fibres of the bundle branches, and in some species those of the bundle itself, are the largest in the heart; they are only locally striated, and possess very large and often multiple nuclei. They are called the fibres of Purkinje. Reaching the muscle of the ventricle, the wave penetrates this everywhere alone lines which radiate from the centres of the two ventricular cavities, and terminates when it reaches the pericardial surface of the heart (4). Of these conclusions also we need have no doubt. the evidence is now too overwhelmingly in their favour. It is important to realize that any studies of conduction in the mammalian heart must take strict account of these anatomical paths. Further, it must be recognized at the beginning that the propagated wave, in its course from the s.-a. node to the ventricular pericardium, passes through fibres of varying architecture and constitution. In its passage from the s.-a. node to the pericardial surface of the ventricle, the wave of the contraction process traverses muscular tissues of four types. These may be described in terms of relative size and of glycogen content. Size of fibre. The largest striated fibres of the heart are the Purkinje cells ; next to these stand the auricular fibres; the ventricular fibres are somewhat smaller ;? smallest of the four are the fibres of the a.-v. node. Glycogen content. Glycogen is held in greatest quantity in the Purkinje cells (5); these are laden. There is a good deal of glycogen in both the auricular and ventricular muscle, and it is said by those who have compared the two (6) that the auricle holds more than the ventricle. The a.-v. node holds little or no glycogen (7). Thus the fibres through which the wave is conducted vary in their structure, they vary in their chemical constitution. Now, when we find tissues which differ in structure, we may be sure that differences in function will also be discovered ; further, when we realize that hand in hand with these structural differences there are parallel differences of chemical constitution, we may anticipate with some confidence differences in function which are arranged in like order. Having considered the size and the glycogen content of the striated fibres of the heart we place these fibres in their natural order, commencing with the fibres which are largest and richest in glycogen and ending with those which are smallest and poorest in glycogen. ‘The order is as follows: 1. Purkinje fibre. 2. Auricular fibre. 3. Ventricular fibre. 4. Nodal fibre. ° The last difference is not emphatic.
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