28 TISSUES AND ORGANS 



rearrangement rather than true new formation of cells : certainly 

 many writers are far from convinced of the existence of such 

 changes during pregnancy. 



MUSCLE 



Controversies regarding the structure of muscular tissue are as 

 old as is the science of histology itself ; and many of the arguments 

 are still far from being concluded. 



It is admitted that striated muscle is made up of threads along 

 which at regular intervals transverse membrane-like structures 

 occur. Further, it seems fairly certain that the threads absorb 

 fluid from the intervening spaces during the process of contraction ; 

 and as a result there is bulging of the threads between the 

 transverse membranes. 



When one considers how similar are the obvious functions of all 

 muscles, it is surprising to find how different is the structure of 

 unstriated muscle from that of the striated variety. It is claimed 

 that in the former there is a distinct chemical change during 

 contraction ; and this is shown by a decreased intensity of staining 

 reaction and a more fibrillar appearance of the fibres. It is 

 regrettable that exactly similar aj^pearances have been described 

 during relaxation, so that all that can safely be affirmed is that 

 chemical changes, manifested as alterations of staining reaction, 

 occur in unstriped muscle at some phase of its activity. 



Cardiac muscle, of course, presents peculiarities of its own. It 

 is now generally admitted that the whole of tlie musculature of 

 the heart is a true syncytium, and, although the information is not 

 all new, we must rapidly review its peculiarities. The fibres 

 possess a sarcolemma outside them and ordinary nuclei inside. 

 Sarcoplasm lies inside the sarcolemma. We do not know the exact 

 diameter or length of the fibres for the obvious reason that their 

 endings cannot be seen. In the uncontracted phase there are to 

 be seen the well-known light and dark discs. The latter contain 

 an anisotropic substance, whilst the former seem to be devoid of 

 this material. The light disc is bisected by a thin line at each 

 end of which is an end-disc. The unit of a fibril appears to be 

 that amount of tissue that lies between two of the thin lines er 



