CONTRACTILITY 33 



fibres. These two classes show certain difierences in form, 

 mode of contraction and function. The structural differ- 

 ences will be dealt with more fully in a subsequent para- 

 graph. It is only necessary to point out here that in 

 unstriated muscles the fibres are beheved to be connected 

 to one another by fine bridges of contractile tissue, the 

 consequence being that a state of contraction is propagated 

 from fibre to fibre throughout the whole muscle. In 

 striated muscle, on the other hand, each fibre receives a 

 nerve filament and is independent of its neighbours. On 

 account of this difference between the two types, an 

 unstriated muscle always contracts as a whole, whereas 

 in striated muscle the contraction can be graded by 

 varying the number of fibres brought into play. 



As to the form of contraction, striated muscle differs 

 from unstriated in its greater rapidity and force of con- 

 traction. The other difference between striated and un- 

 striated muscle lies in their relation to the central nervous 

 system. The striated are usually, but not always, under 

 the control of the will. The unstriated are not directly 

 under voluntary control ; they usually subserve visceral 

 functions. The striated, highly speciahsed though they 

 are in contractile power, are incapable of any form of con- 

 traction except in obedience to impulses arriving from the 

 nervous centres, and, owing to a constant flow of impulses, 

 they are normally in a condition of partial contraction or 

 tonus. Cut off from these impulses, they become flabby 

 or toneless. Unstriated muscles on the other hand 

 have in large measure retained a power of contraction 

 independent of outside influences. Like the striated, they 

 are normally in a state of tonus, but the tonus is an inherent 

 property of the muscles themselves, being independent of 

 impulses arriving from the nervous centres. Besides tonus, 

 they often possess a power of rhythmic contraction, an 

 example of which is seen in the muscle of the intestinal 

 wall. But though capable of contraction independently 

 of the nervous system, their tonus and rhythm are still 

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