MYOCARDIUM 29 



discs ; these are the telopliragmata of Hcidcnhain. There are 

 many more details of structure described by various writers, but 

 we have here enough for our present needs. Apart from the 

 syncytial arrangement, which differentiates the myocardium from 

 ordinary striated muscle, there is another striking difference. The 

 intercalated discs are transverse structures that are found at 

 intervals of about forty units or sarcomeres. In healthy muscle 

 these are straight and lie right across the w-idth of the fibre, but 

 some travel only a part of the distance, even as little as a quarter. 

 More complicated intercalated discs have a fairly regular step-like 

 arrangement. It is probable that the more complicated forms are 

 accompaniments of advancing age. The most difficult problem is 

 that of the reversal of striation during contraction, and it is still 

 impossible to give an explanation of this occurrence. 



The specialisation of contractile function of the bundle of His 

 has led to intensive morphological studies of this vital structure, 

 and, although a good deal is known, the chapter is far from com- 

 plete. There are three forms of Purkinje cells : some spheroidal 

 with abundant protoplasm and a striated border, others 

 elongated in form with more obvious striation and less bulky 

 cytoplasm ; and even more elongated elements with striation 

 occupying almost the w^hole thickness of the cell. The last 

 variety of cell appears to become continuous with the ordinary 

 myocardium. The cells are stated to possess four layers : a 

 central zone enclosing the nuclei ; a granular layer around this, in 

 which the granules stain intensely with iron hsematoxylin ; a 

 stratum with very fine longitudinal striation and practically no 

 transverse striation ; and a striated cortex in which the fibrils run 

 circularly, obliquely or longitudinally. 



In the human heart it is now customary to distinguish cells of 

 embryological appearance from others of adult type. The former 

 are large polyhedral elements containing several nuclei near their 

 centre. The perinuclear zone is granular, w^hereas the periphery 

 of the cell is fibrillar. Some of these cells lie, apparently as 

 association mechanisms, between adjacent bundles. Most striking 

 is the fact that these intercalated nodes often form a plexiform 

 arrangement such as that described by Tawara, and now usually 



