MUSCULAR TISSUE. 97 



the canula withdrawn. The bladder is cut out, still dis- 

 tended, and put in the same bichromate solution, where 

 it remains for three days, when it is washed and trans- 

 ferred to alcohol. After twenty-four hours it may be 

 opened, and a bit cut out, the epithelium carefully 

 brushed from the inner surface, and stained double 

 and mounted in glycerin. "In addition to the muscle- 

 cells, the nuclei of the endothelial cells covering the 

 peritoneal surface will be seen, as well as connective 

 cells and fibres in the wall of the bladder. 



2. STRIATED VOLUNTARY MUSCULAR TISSUE. 



As the involuntary striated or heart muscle occu- 

 pies, in structure, an intermediate position between 

 the smooth and the voluntary striated muscle, we 

 shall find it advantageous to postpone its study until 

 we have considered the other varieties. 



Voluntary striated muscle, to which the greater 

 part of the muscular tissue of the body belongs, is 

 made up of narrow, cylindrical, cord-like elements, 

 of varying length and thickness, called muscular 

 fibres. These are grouped in variously shaped 

 bundles or fascicles, surrounded by connective- 

 tissue envelopes or sheaths, and abundantly sup- 

 plied with blood-vessels and nerves. Let us first 

 study the structure of the individual fibres a They 

 consist of three distinct elements: I, contractile 

 substance, forming the centre and making up most 

 of the bulk of the fibre ; 2, nuclei, which in man 



