242 



GENERAL PHYSIOLOGY 



the delicate Vorticella, possess a single, smooth muscle-fibre, com- 

 posed of several fibrillse cemented together ; this extends outside 

 the body as a thick strand and, surrounded by an elastic sheath 

 to the inner wall of which it is fixed in an elongated spiral, serves 

 the cell-body as a stalk for attachment (Fig. 101). In smooth 

 muscle-cells that are united in the cell-community to form tissues, 

 the protoplasmic body is reduced very much in quantity in com- 

 parison with the contractile fibrillae. It either forms merely a 

 small sarcoplasmic mass containing the nucleus, which is enclosed 

 by a long, spindle-shaped covering of contractile, fibrillar substance, 



Fig. 101. — VoHicella. a, Extended; b, con- 

 tracted (the stalk-muscle is not seen in 

 a and 6) ; c, stalk-sheath containing 

 muscle-fibre, strongly magnified. 



P'IG. 102. — Smooth muscle-cells 

 — a, from the bladder of the 

 frog ; 6, from the retractor 

 muscles of fresh-water Bri/o- 



as in the smooth muscle-cells from the bladder of the frog 

 (Fig. 102, a), or it lies as a small cell-body in the middle, lateral to 

 the contractile bundle of fibrillte, as in the retractor muscles of 

 fresh-water Bryozoa (Fig. 102, 6). 



The structure of cross-striated muscle-fibres is far more complex. 

 As a type of these, which, like the smooth muscle, appear 

 in manifold modifications, the insect muscle-fibre may serve, 

 the structure of which has become known in minute detail, 

 especially through the striking and extended investigations of 

 Engelmann and recently RoUett. The cross-striated muscle-fibre 



