MUSCULAR MOVEMENTS. 



465 



In the contractile sheets formed of involuntary muscular tissue, the fibres 

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



FIG. 148. Muscular fibres 

 from the aorta of the 

 calf ; magnified 200 

 diameters (Sappey). 



1,1, fibres joined with each 

 other ; 2, 2, 2, isolated 

 fibres. 



FIG. 149. Muscular fibres from the uterus of 

 a woman who died at the ninth month of 

 vterogestation ; magnified 350 diameters 

 (Sappey). 



1, 1,2, short, wide fibres : 3, 4, 5, 5. longer and 

 narrower fibres ; 6, 6, two fibres united 

 at 7 ; 8, small fibres in process of develop- 

 ment. 



FIG. 147. Muscular fibres 

 from the urinary blad- 

 der of the human subject; 

 magnified 200 diameters 

 (Sappey). 



1, 1, 1, nuclei ; 2, 2, 2, bor- 

 ders of some of the 

 fibres : 3, 3, isolated 

 fibres ; 4, 4, two fibres 

 joined together at 5. 



it were, dove-tailed into each other. Generally 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 very smallest arterioles. Usually the layers 

 are multiple, being superimposed in regular order. The action of acetic acid 

 is to render the fibres pale so that their outlines become almost indistin- 

 guishable, and to bring the nuclei more distinctly into view. 



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

 tion of the involuntary muscles is peculiar. It does not take place immedi- 

 ately upon 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 intes- 

 tines gives an idea of the mode of contraction of these fibres, with the grad- 

 ual propagation of the stimulus along the alimentary canal as the food makes 

 its impression upon the mucous membrane. Another illustration is afforded 

 by labor-pains. These are due to the muscular contractions of the uterus, 

 and they last for a few seconds or one or two minutes. Their gradual access, 

 continuation for a certain period, and gradual disappearance coincide with 

 the history of the contractions of the involuntary muscular fibres. 



The contraction of the involuntary muscular tissue is slow, and the fibres 

 return slowly to a condition of repose. The movements are always involun- 



