41 8 MOVEMENTS 



the alimentary canal, has given them the name of organic muscular 

 fibres, or fibres of organic life. In their natural condition, the invol- 

 untary muscular fibres are pale, finely granular, flattened, and of an 

 elongated spindle-shape, with a very long, narrow, almost linear nucleus. 

 The nucleus usually has no distinct nucleolus and sometimes it is curved 

 or shaped like the letter S. The ordinary length of these fibres is 

 about ^-Q of an inch (50 /*) and their breadth about ^Fo f an mc ^ 

 (6 p). In the gravid uterus they undergo remarkable hypertrophy, 

 measuring here -^ to -fa of an inch (300 to 500 ft) in length, and 2Wo 

 of an inch (12 /*) in breadth. 



In the contractile sheets of involuntary muscular tissue, the fibres 

 are arranged side by side, are closely adherent, and their extremities 

 are, as it were, dove-tailed into each other. Usually the borders of the 

 fibres are regular and their extremities are simple; but sometimes the 

 ends are forked and the borders present one or more little projections. 

 The fibres seldom exist in a single layer except in the smallest arterioles. 

 Usually the layers are multiple, being superimposed in regular order. 



Contraction of the Involuntary Muscular Tissue. The mode of con- 

 traction of the involuntary muscles is peculiar. It does not take place 

 immediately on the reception of a stimulus, applied either directly or 

 through the nerves, but it is gradual, enduring for a time and then 

 followed by slow and gradual relaxation. A description of the peristaltic 

 movements of the intestines gives an idea of the mode of contraction 

 of these fibres, with the gradual propagation of the stimulus along the 

 alimentary canal as the food makes its impression on the mucous mem- 

 brane. Another illustration is afforded by labor-pains. These are due 

 to the muscular contractions of the uterus, and they last for a few sec- 

 onds or one or two minutes. Their gradual access, continuation for a 

 certain period, and gradual disappearance coincide with the description 

 of contractions of the involuntary muscular fibres. 



The contraction of the involuntary muscular tissue is relatively slow, 

 and the fibres return slowly to a condition of repose. The movements 

 are always involuntary. Peristaltic action is the rule, and the contrac- 

 tion takes place progressively and without oscillations. Contractility 

 persists for a long time after death. Excitation of the nerves has less 

 influence on contraction of these fibres than direct excitation of the 

 muscles. The involuntary muscular tissue is regenerated rapidly, while 

 the structure of the voluntary muscles is restored with difficulty after 

 destruction or division. 



Physiological Anatomy of the Voluntary Muscular Tissue. A vol- 

 untary muscle contains, in addition to its contractile substance, fibres of 

 inelastic and elastic tissue, adipose tissue, abundant bloodvessels, nerves 



