492 Dynamic Theory. 



the will. The Smooth muscles consist of elongated, band-like fibres free 

 from striation. In the intestines which are moved by smooth muscles, 

 some of the fibres are longitudinal and others transverse or ring-like. 

 The fibres of the striated muscles are arranged in bundles or fasciculi, 

 which are enveloped in and bound together by areolar or connective tis- 

 sue, which also supports the nerves and vessels of the muscles. 



FIG. 231 Striated Muscle Fibres. 



1. Two fibres cut off at the left and 

 running into tendons at the right. 



2. Cross section of a fibre, showing 

 ends of fibrillEe and areolar connective 

 tissue. 



3. Fibre pulled apart forming discs. 



4. Fibre split up into its fibrillse at 

 one end. 



5, 6. Single fibrillae. 



There are about 625 fibrilla3 in a fibre. 



Each fibre of the bundle above 

 described is enveloped in a 

 sheath called the sarcolemma. 

 This is a membrane, transparent 

 and very delicate, but tough and 

 FIG 231 elastic, and it separates the fi- 



bres from each other and from all other tissues. It is not penetrated 

 by nerves or blood vessels. On account of the pressure of the fibres 

 upon each other their shape is polygonal rather than round or cylin- 

 drical. They will average in diameter about ^ of an inch. Each fibre 

 is susceptible to a still further subdivision into minute threads called 

 ' fibrillce or fibrils. Each of these fibrils is composed of a series of dark- 

 colored cells, each cell surrounded and enclosed by a pellucid border, 

 the effect being to give the fibril the appearance of a string of dark- 

 colored beads separated by light-colored partitions. These light-colored 

 partitions in any one fibril come directly opposite those in the other 

 fibrils, which circumstance gives the appearance of the transverse striae 

 across the fibre above spoken of. If a fibre be pulled apart by a longi- 

 tudinal strain it is apt to divide in these transverse markings, and so 

 separate into discs or plates, each disc composed of one cell from each 

 fibril. (Fig. 231.) When the muscle is relaxed, the beads or cells 

 which constitute the fibrils are elongated ; their longitudinal diameter 

 being greater than the transverse. But when it is contracted, the fibril 

 cells are squeezed together endwise and the transverse diameter becomes 

 greater. Each of the ultimate strands or fibrils, therefore, acts under 

 stimulus in a manner which closely imitates the behavior of the melted 

 metallic conductors spoken of above. The cells which compose the 

 fibrils are therefore polarized units, which, by their mutual polar attrac- 

 tions shorten, and consequently widen when under stimulus, the united 

 action causing the general shortening and widening of the whole mus- 

 cle. This also corresponds with the contraction of certain vegetable 



