530 Caroline McGill. 



observed and described by earlier workers was as a rule undoubt- 

 edly brought about by removal of the tension under which the fibers 

 normally contract. They studied only small isolated bits of tissue, 

 so their results are easily explained. 



As the myofibrillse enter the contraction nodes they often appear 

 to thicken distinctly, Figs. 5, 6, 35, 40. The amount of thicken- 

 ing depends upon the amount of contraction. The nodes labeled a 

 in Figs. 39, 40, are just beginning to contract, so there is only a 

 slight increase in diameter of the myofibril^. Nodes labeled h in 

 Figs. 35, 40, are more firmly contracted and show more marked 

 enlargement. 



In muscle where the contraction nodes come very close together, 

 as in pig's oesophagus when the tissue is just beginning to contract, 

 the myofibrillse may appear segmented, as shown in Fig. 26. Even 

 in wide contraction nodes, as in those of arteries, the contracting 

 myofibrillffi may thicken unequally, giving a segmented appear- 

 ance, Fig. 18, from the carotid of ox. Where this condition is 

 marked it may give an appearance closely simulating the myofi- 

 brillse of striated muscle. The segmentation of the myofibrillae of 

 the smooth muscle in the mesentery of Urodela, described by 

 Schaper, 1902, was probably due to mild contraction. 



However, it should be remembered that iron-haematoxylin and 

 orange G, the only stains by which the myofibrill^ of contracted 

 muscle could be demonstrated, are largely physical stains. Instead 

 of there being a true enlargement of the myofibrillse themselves at 

 the point of contraction, the physical condition of the inter-fibrillar 

 substance immediately surrounding the myofibrillae may be so altered 

 as to make it stain as intensely as the myofibrillse, and thus pro- 

 duce an apparent though not a real thickening of the fibrillae. Were 

 the physical condition of all the inter-fibrillar substance so altered, 

 the stained material would show the whole contraction node homo- 

 geneous. On the other hand, the homogeneous appearance of con- 

 traction nodes may be due to the crowding together of enlarged 

 myofibrillae. If closely packed, in material deeply stained, the 

 myofibrillae would not be demonstrated. 



