TISSUES OF SPECIAL FUNCTION. 



87 



so thin as often to escape observation. These differences render it 

 easy to distinguish the two kinds of tissue in spite of their general 

 similarity when seen in cross-section. 



It is, of course, rarely that sections contain smooth muscular 



FIG. 71. 



FIG. 72. 



a bed 



if ii 

 a' b' c' d 



Diagrams of smooth muscular fibres cut in various directions. 



Fig. 71. Fibres cut exactly perpendicular to their long axes. The lines A A and B B in the 

 upper figure indicate the portions of the fibres included in the section, which is viewed 

 from above in the lower figure. The cross-sections of the fibres a and & contain cross- 

 sections of the nuclei ; those of c and d are smaller and devoid of nuclei. 



Fig. 72. Fibres cut obliquely to their long axes. When the upper surface of the section, 

 marked by the line A A in the upper figure, is in sharp focus, the sections of fibres appear 

 as in a, b, c, and d of the lower figure. When the bottom of the section, indicated by the 

 line B B in the upper diagram, is in clear view, the sections of the fibres appear as shown 

 at a', V, c', and d'. It will be noticed that the optical section of fibre a in the upper dia- 

 gram has moved from a to a' in the lower diagram. As the focus was changed, the nucleus 

 in fibre a was constantly present and its optical section appeared of uniform size. This 

 could only be the case when the nucleus was rod-shaped and of the same diameter 

 throughout that portion contained in the section. In the fibre d the nucleus was visible 

 when the upper surface of the section was in focus, but disappeared when the focal plane 

 was depressed. 



