280 



SPECIAL HISTOLOGY. 



Fig. 108. 



§ 87. 



Physiological Remarks. — The most remarkable peculiarity of 

 the muscles is their contractility . In each contraction, the 

 primitive fasciculi shorten themselves in a rectilinear direc- 

 tion, and at the same time become thicker ; they do not, how- 

 ever, undergo any considerable condensation. It is probable, 

 that the contractions generally take place simultaneously in 

 every part of a fasciculus, although at the same time it is not, 

 of course, intended to be said, that the contraction does not 

 commence at the points where the terminations of the nerves 

 occur, and that this contraction does not precede, though by a 

 space of time immeasurably short, or at all events inappre- 

 ciable by the eye, that of the other portions of the fasciculus. 

 Under certain conditions, however, successive, progressive, and 

 partial contractions are observed. If, during the 

 contraction of a muscle, its longitudinal and trans- 

 verse stria? are noticed, it is not difficult to show 

 that where the former exist, they disappear during 

 the contraction, and give place to transverse mark- 

 ings; and that the latter, where they were already 

 present, become more distinct, and more closely 

 approximated. Moreover, in the easily isolated fibrils 

 of the thoracic muscles of insects, it is easy to per- 

 ceive that they exhibit very variable conditions in 

 different animals, and vaiy frequently in one and 

 the same individual. Sometimes they are almost 

 without transverse markings, and very pale ; some- 

 times darker, and with distinct transverse lines; some- 

 times, again, very distinctly ringed ; and, together 

 with these varying conditions, does the thickness 

 of the fibrils and the distance between the transverse 



versely almost ceased, while the transverse marking became less distinct, and the 

 fibres broke up very readily into fibrils (fig. 2). In this condition they often resem- 

 bled the smooth muscles of mollusks. 



Fig. 108. Primitive fibres from the alar muscles of the " Dung-Fly," x 350 diam.: 

 a, slender fibril, with very distant delicate transverse striae; b, thicker fibre, with 

 closer, alternately stronger and fainter stria' ; c, still thicker fibril, with the striae 

 more closely approximated; d, fibril with lateral alternate elevations (they have 

 come out too dark). 



