:\IUSCLE TISSUE 125 



of the cell around the nucleus is wholly composed of these little 

 discs of sarcoplasm. 



//Peculiar to heart muscle are what appear in longitudinal sections 

 to' be dark lines which cross transversely both main fibres and side' 

 branches. These arc known as itilcrcallated discs and dixidr the 

 muscle fibre into irregular, short, thick cylindrical segments (Fig. 69). 

 A disc may pass straight across a fibre, or it may cross it in a series of 

 steps, or it may extend only part way across the fibre. It always 

 touches at some point one of the ground membranes^ and sometimes 



r' - ■ . ■ ■ ^ -■•■ , , 





!-l4;n-:fe?'; ^!v>^': 



■1 



Fig. 68. — Section of Heart Muscle. X350. (Technic 7, p _ 129.) a, Cells cut 

 longitudinally; b, cells cut transversely (only three nuclei have been included in the plane 

 of section); c, cells cut obliquely; d, connective-tissue septum. 



entirely fills the space between two ground membranes which lie 

 unusually close together. Special technic has demonstrated the 

 fact that the fibrillae do not stop at the disc but are continuous 

 through it, although they show in passage some modification of 

 structure. 1/ 



The one question which has been most discussed in regard to the structure 

 of heart muscle and which remains still unanswered is whether heart muscle is 

 cellular or a syncytium. The solution of the problem is of course dependent 

 upon the determination of the significance of the intercallated discs — whether or 

 not they represent true cell boundaries. The segments into which the heart 

 muscle fibre is divided by the discs have long been described as cells. The fact 

 that the nuclei are usually placed about midway between two discs; that the 

 discs show the same staining reaction as intercelltilar substance when subjected 

 to the action of silver nitrate, and the ease with which heart muscle may be 



1 Some authorities deny this. 



