STRUCTURE OF THE INVOLUNTARY MUSCLES. 



527 



Physiological Anatomy of the Involuntary Muscles. We have so often described this 

 tissue, as it is found in the vascular system, the digestive organs, the skin, and in other 

 situations, that it will not be necessary, in this connection, to give more than a sketch 

 of its structure and mode of action. 



The involuntary muscular system presents a striking contrast to the voluntary mus- 

 cles, not only in its minute anatomy and mode of action, but in the arrangement of its 

 fibres. While the voluntary muscles are almost invariably attached by their two extrem- 

 ities to movable parts, the involuntary muscles form sheets or membranes in the walls of 

 hollow organs, and, by their contraction, they simply modify the capacity of the cavities 

 which they enclose. Various names have been given to this tissue to denote its distribu- 

 tion, mode of action, or structure. The name involuntary muscle indicates that its contrac- 

 tion is not under the control of the will ; and this is the fact, these muscles being chiefly 

 animated by the sympathetic system of nerves, while the voluntary muscles are supplied 

 mainly from the cerebro-spinal system. On account of the peculiar structure of these 

 fibres, they have been called muscular fibre-cells, smooth muscular fibres, pale fibres, 

 non-striated fibres, fusiform fibres, and contractile cells. The distribution of these fibres 

 to parts concerned in the organic or vegetative functions, as the alimentary canal, has 

 given them the name of organic muscular fibres, or fibres of organic, or vegetative life. 

 It is difficult to isolate the individual fibres of this tissue in microscopical preparations; 

 and, when seen in situ, their borders are faint, and we can make out their arrangement 



FIG. 151. Muscular fibres from 

 the urinary bladder of the 

 hitman subject; magnified 

 200 diameters. (Sappey.) 



1, 1, 1, nuclei ; 2, 2,. 2, borders of 

 some of the fibres ; 3, 3, iso- 

 lated fibres; 4, 4, two fibres 

 joined together at (5). 



FIG. 152. Muscular fibres from 

 the aorta of the calf; magni- 

 fied 200 diameters. (Sappey.) 



1. 1. fibres joined with each other ; 

 2, 2, 2, isolated fibres. 



FIG. 153. Muscular fibres from the uterus 

 of a woman who died at the ninth 

 month of utero-gestation ; 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 development. 



best by the appearance of their nuclei. Robin recommends soaking of the tissue for a few 

 days in a mixture of one part of ordinary nitric acid to ten of water. This renders the 

 fibres dark and granular, makes their borders very distinct, and frequently some of them 

 become entirely isolated. The nuclei, however, are obscured. In their natural condi- 

 tion, the fibres are excessively pale, very finely granular, flattened, and of an elongated 

 spindle-shape, with a very long, narrow, almost linear nucleus in the centre. The nu- 

 cleus generally has no nucleolus, and it is sometimes curved or shaped like the letter S. 



