MINUTE ANATOMY OF THE HEART. 253 



of the heart * The fibres are united to one another in the form 

 of networks, and are composed of more or less prismatic segments 

 (granules), having a diameter of from 0*05 to O10 millimeters^ 

 each of which consists of a cortical layer of transversely striated 

 fibrillar muscle substance, and a hyaline axile material con- 

 taining one or two clear nuclei. Some observers regard the 

 transversely striated mass as an intermediate substance, within 

 which are deposited transparent isolated cells; whilst others, with 

 whom I agree, regard each granule as a muscle cell, in which 

 (as in a certain stage of development) only the peripheric 

 layers have undergone conversion into contractile substance. 



In what relations these segments of the fibres of Purkinje 

 stand to the cardiac muscle in its fully developed condition, is 

 a subject that can only be elucidated by a knowledge of the 

 history of development; but it may here be remarked that 

 various observations have been made, which agree in showing 

 that the fibres of Purkinje pass directly into ordinary muscular 

 bands, and that in some animals their place can be supplied by 

 ordinary muscular tissue. The controversy whether this or 

 that animal possesses the fibres of Purkinje is therefore of 

 small importance, because the differences depend merely upon 

 the various forms presented by the endocardial muscle. 



A division of the stronger fibres, as we have already seen, 

 does not occur here, whilst they are for 'the most part surrounded 

 by a well-marked sheath of connective tissue. These sheaths, 

 when penetrated by an injection pipe, sometimes become filled 

 with injection, and then form a wide-meshed network which 

 it is impossible to mistake for the vessels of the lymphatic 

 system (Eberth). 



As already indicated in discussing the internal membrane of 

 the heart, we have to consider the valves. These indeed are 

 usually considered to be duplicatures of the endocardium, 

 but this expression is not absolutely correct. The substance 



* Besides the work of Purkinje (Miiller's Archiv, 1845, p. 294), reference 

 may be made to the statements of Kolliker, Hessling, Reichert, Remak, 

 Aeby, Obermaier, and Lehnert. More exact and extended references to the 

 literature of the subject will be found in the last-named authors. Obermaier, 

 Archiv fur Anatomie u. Physiologic, 1867, pp. 245 u. 358 ; Lehnert, Max 

 Schultze's Archiv fur Mikroskopische Anatomie, 1868, p. 26. 



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