SEC. 7. ON SOME OTHER FORMS OF CONTRACTILE 



TISSUE. 



Plain, Smooth or Unstriated Muscular Tissue. 



89. This, in vertebrates at all events, rarely occurs in isolated 

 masses or muscles, as does striated muscular tissue, but is usually 

 found taking part in the structure of complex organs, such for 

 instance as the intestines ; hence the investigation of its properties 

 is beset with many difficulties. 



It is usually arranged in sheets, composed of flattened bundles 

 or bands bound together by connective tissue carrying blood vessels, 

 lymphatics and nerves. Some of these bundles or bands may be 

 split up into smaller bands similarly united to each other by con- 

 nective tissue, but in many cases the whole sheet being thin is made 

 up directly of small bands. Each small band is composed of a 

 number of elementary fibres or fibre cells, which in a certain sense 

 are analogous to the striated elementary fibres, but in many respects 

 differ widely from them. 



Each unstriated elementary fibre is a minute object, from 50 ^ 

 to 200 /ji in length and from 5 /A to 10 /j, in breadth ; it is therefore, 

 in size, of a wholly different order from a striated fibre. It is fusi- 

 form or spindle-shaped, somewhat flattened in the middle and 

 tapering to a point at the ends, which in some cases are branched ; 

 but the exact form of the fibre will differ according as the muscle 

 is in a state of contraction or relaxation. 



Midway between the two ends and in the centre of the fusiform 

 body lies a nucleus, which in a normal condition is elliptical in out- 

 line, with its long axis lying lengthwise, but which under the 

 influence of reagents is very apt to become rod-shaped ; hence in 

 prepared specimens the presence of these rod-shaped nuclei is very 

 characteristic of plain muscular tissue. 



The nucleus has the ordinary characters of a nucleus, and very 

 frequently two nucleoli are conspicuous. Around the nucleus is 



