88 



NORMAL HISTOLOGY. 

 FIG. 73. 



Diagrams of smooth muscular fibres cut very obliquely. The explanation of Fig. 72, already 

 given, will make this one clear. In this case the outlines of the fibres in section will be 

 less sharply defined than in the preceding case, because, for instance, at the point a the 

 fibre a' is cut so as to leave only a thin edge, difficult of detection, and the fibre b has had 

 such a thin slice removed from it that the loss would be hardly perceptible. The appear- 

 ance of the section would, therefore, be much less easy of interpretation than is repre- 

 sented in the lower figure, where the outlines of the sections are equally distinct 

 throughout. 



tissue cut exactly in either of the directions just considered. In the 

 majority of sections that come under observation the muscle-fibres 

 are cut obliquely, and the oval or polygonal areas which result are, 

 therefore, elongated. The nuclei of the cells lie at an angle with 

 the line of vision, and, in consequence, appear foreshortened. If 

 now we focus the instrument so as to get a sharp image of the upper 

 surface of the section, and then rapidly turn the fine adjustment so 

 as to bring the lower surface into focus, we shall notice an apparent 

 lateral motion of the nuclei and cell-sections. This apparent lateral 

 movement, due to change of focus, is an evidence of the elongated 

 shape and oblique position of the objects exhibiting it ; and a little 

 reflection will convince the student that such oblique sections, when 

 carefully studied, are better calculated to reveal the shapes and rela- 

 tive positions of the tissue-elements than either perfect longitudinal 

 or cross-sections. He should seek to train his powers of observa- 

 tion so that he may readily interpret the instructive, though at first 

 confusing, images presented by such sections (Figs. 72 and 73). 



Smooth muscular tissue is not under the direct control of the will. 

 For this reason it is frequently called " involuntary muscle." It is 

 also sometimes designated as non-striated or unstriped muscle, in 

 contradistinction to the other two varieties of muscular tissue, the 

 fibres of which present distinct cross-striations. 



The functional contractions of smooth muscular fibres are slug- 

 gish. The fibres are slow in responding to stimulation, contract 

 leisurely, maintain the contracted condition for a long time, and 

 then gradually relax. These properties render the tissue of value 



