338 THE SOLIDS. 



somewhat flat, and which contain imbedded in their substance, 

 elongated, and granular nuclei. (See Plate XLI. Jig. 2.) 



The fibrillas usually run parallel to each other, and form 

 thin layers and fasciculi, which are separated from each 

 other by cellular tissue, and frequently interlace. 



The nuclei are sometimes imbedded in the substance of 

 the fibrillae, without at the same time increasing their dia- 

 meter : at others they render the fibrillae ventricose from 

 their great size ; and again, in other cases, they protrude 

 from their sides. (See Plate XLI. fig. 2.) They are best 

 seen after the addition of acetic acid. 



Unstriped muscles are doubtless freely supplied with 

 blood-vessels and with nerves. 



The unstriped muscle is called into action with greater 

 difficulty than the striped ; its action is also slower, and of 

 a peculiar kind, giving rise to the vermicular and peristaltic 

 motion, seen especially in the intestines. 



This slower and less energetic action results from its lower 

 degree of organization. 



The muscular structure of the heart, the action of which is 

 to a considerable extent involuntary, requires a special de- 

 scription. The muscular tissue of this organ has been usually 

 supposed to constitute an exception in its structure to that of 

 other involuntary muscles, and that while it performed the 

 office of an involuntary muscle, it yet possessed the structure 

 of a voluntary muscle, its fibres being striped. 



Mr. Bowman, one of the very best authorities on the struc- 

 ture of the muscular fibre, gives the following description of 

 the tissue of the heart. " The cross stripes on the fibres of 

 the heart are not usually so regular or distinct as in those of 

 the voluntary muscles. They are often interrupted, or even 

 not visible at all. In some of the lower animals their sarcous 

 elements never form transverse stripes. These fibres are 

 usually smaller than the average diameter of those of the 

 voluntary muscles of the same subject by two-thirds, as stated 

 by Mr. Skey." * 



This description is very imperfect, and in some respects, 

 * Physiological Anatomy, p. 161. 



