THE SINO- VENTRICULAR BUNDLE. 153 



the bundle-fibers contain a larger amount of glycogen than do cardiac fibers. But 

 the ratio seems to be in the same proportion as the amount of sarcoplasm. 



A study of the lipoid, protein, and mitochondria content can not be dissociated. 

 Mitochondria or chondriosomes are morphological constituents of the cell, of 

 whose chemical nature we know but little. The lipoid elements may appear in 

 the form of granules, liposomes, or may be attached to or a part of the mitochon- 

 dria. As to the protein, myogen, and myosin, we may say that it can be either in 

 soluble form and optically inactive or attached to a part of some of the morphologi- 

 cal constituents which have been lumped under the term interstitial granules. We 

 must make a distinction, however, between the granules which are situated between 

 the fibrils or sarcostyles and those that are found in the perinuclear space. My 

 work has led me to a closer analysis of these structures, but I can not say that 

 the results are conclusive. I therefore make the following statements with some 

 reservation : 



Some interfibrillar granules seem to bear the same relation to the fibrils, in 

 regard to both size and structure, in the bundle fibers as they do in cardiac muscle. 

 I am inclined to consider them as contraction phases in Holmgren's (1910) sense. 

 There are others which have been called mitochondria, but as they do not give all 

 the mitochondrial reactions I am doubtful of their nature. Mironesco (1898) has 

 found, however, that a large amount of mitochondria are present in the Purkinje 

 fibers; he concludes that the latter represent a reserve material for cardiac mus- 

 culature. Duesberg (1910) has shown (and so far his work has not been disproved) 

 that myofibrils arise from mitochondria or chondriosomes. The perinuclear gran- 

 ules, however, are both larger and more numerous in the bundle, especially in the 

 terminal portion, than they are in cardiac muscle. Some of these granules give the 

 glycogen reaction, while others show that they must be lipoid in nature. 



FUNCTIONAL INTERPRETATION. 



The object of this paper is to give a physiological interpretation of the facts 

 that are brought forth by careful anatomical observations. We realize that to a 

 great extent the study of function belongs to the physiological laboratory; but 

 when we realize that in former years the physiologists were also the histologists, 

 and that furthermore much of our early physiological knowledge was derived 

 purely from microscopical studies, it may not be amiss to again use our anatomical 

 knowledge for the explanation of physiological phenomena. In doing so we fully 

 realize that it is speculative, but it has its justification if it can prove of value in 

 directing the efforts of the experimentalist. 



McCallum (1898) stated that the ultimate number of muscle-fibers in volun- 

 tary muscles is laid down at birth. Increase in the amount of musculature after 

 this time is due to the increase in the number of fibrils. The same was held to be 

 true for the muscles of the heart. It seems inconceivable to me that muscle-cells 

 should differ fundamentally from all other cells which have even a less constant 

 activity. We know that gland cells are replaced. Why should not muscle-fibers 

 be replaced? This question seems especially pertinent in reference to the heart. 



