INVOLUNTARY MUSCLES. 



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 

 best by the appearance of their nuclei. Robin recommends 

 soaking the tissue for a few days in a mixture of one part of 

 ordinary nitric acid to ten of water. 1 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 condition, 

 the fibres are excessively pale, very finely granular, flat- 

 tened, and of an elongated spindle-shape, with a very long, 

 narrow, almost linear nucleus in the centre. The nucleus 

 generally has no nucleolus, and it is sometimes curved, 

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

 fibres is about -g-J-g-, and their breadth about ^nnnr f an 

 inch. In the gravid uterus they undergo remarkable hyper- 

 trophy, measuring here from -V to -^ of an inch in length, 

 and 20*0 of an inch in breadth. 8 The peculiarities of their 

 structure in the uterus will be fully considered under the 

 head of generation. 



In the contractile sheets formed of the involuntary mus- 

 cular tissue, the fibres are arranged side by side, closely ad- 

 herent, and their extremities, as it were, dove-tailed into 

 each other. Generally the borders of the fibres are regular 

 and their extremities simple; but sometimes the ends are 

 forked, and the borders present one or more little projec- 

 tions. It is very seldom that we see the fibres 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 indistinguishable, and to bring 



1 ROBIN, Recherches sur quelques particularites de la structure des capittaires de 

 Fencephale. Journal de la physiologic, Paris, 1859, tome ii., p. 541. 



2 POUCHET, op. cit., p. 65. 



