TISSUES OF SPECIAL FUNCTION. 



93 



-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. 75 and 76). 



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 



Fig. 77. 



Diagrams of cardiac muscular tissue. 



./I, longitudinal section: a, nucleus of muscle-cell ; 6, unmodified cytoplasm; c, contractile 

 substance with longitudinal and transverse striations ; d, cement-substance uniting con- 

 tiguous cells; e, areolar tissue (vessels omitted) between the muscle-fibres formed by the 

 union of the individual cells ; /, small bloodvessel within the areolar tissue. If the lines 

 of junction between the cells were not visible, the tissue would appear as though com- 

 posed of interlacing and anastomosing fibres, none of which could be traced for any con- 

 siderable distance. Such is the usual appearance of longitudinal sections of cardiac 

 muscle. 



B, transverse section : a, section of a cell, including the nucleus ; c, section above the nucleus 

 and just below a crotch formed by the divergence of a branch; 6, section above the 

 nucleus and the point where the branch 6' is given off. 



leisurely, maintain the contracted condition for a relatively long 

 time, and then gradually relax. These properties render the tissue 

 of value in conferring " tone " to certain structures in which it is 

 found, notably the walls of the arteries and veins. They also 

 render it of service in producing the vermicular movements that are 



