HISTOGENESIS AND STKUCTUEE 



97 



expresses itself in an alternation of light and dark disks ('bands/ 'seg- 

 ments/ 'stripes'), said to consist of iso tropic and anisotropic substances 

 respectively. While the disks are conspicuous both in fresh and stained 

 tissue, the demonstration of their physical properties under the polari- 

 scope is a matter of difficulty. Indeed with crossed Nichols the entire 



FIG. 105. CARDIAC MUSCLE OF THE HUMAN HEART; THE ABUNDANT BRANCHES 



ARE PLAINLY SHOWN. 

 Longitudinal section. Hematein and eosin. Photo. X 120. 



fiber appears lighter than the field, showing the presence of anisotropic 

 materials (granules) scattered throughout the fiber, but a definite band- 

 ing corresponding to the light and dark disks of fresh muscle is not 

 apparent in all striped muscle. It seems more probable that, though 

 anisotropic granules are more abundant in the dark disk, they are not 

 absent in the lighter disk; moreover, they are more or less definitely 

 aggregated in the dark disk according to the phase of contraction. The 



