~>32 A TEXTBOOK OF PHYSIOLO<;V 



Smooth Muscle. A smooth muscle fibre develops from a single 

 cell with a single nucleus. Such cells specialize in the embryo out of 

 the mesenchyme. Fibrils are deposited within the cell, sometimes 

 around the nucleus, sometimes to one side of it. In the latter ca.se, 

 the nucleus may appear on the side of the cell. The fibrils are homo- 

 geneous, and vary in number. There is a considerable amount of 

 sarcoplasm in the smooth muscle fibres, which generally take the form 

 of elongated spindles with thin tapering ends. Occasionally, as in the 

 aorta of young mammals, the ends may be branched. 



Cardiac Muscle. The structure of cardiac muscle has been dealt 

 with in the section on the circulatory system (p. 122). 



_ . ^^^^^^^^^_ The Physical Properties of Muscle. 



In rigor I -, , . 



In speaking of muscle, we gener- 

 ally mean striated muscle, since this 

 is the kind of muscle which forms 



in tetanus H^^^^H^^^^H ' ' )(> nesn ;U1( ' nas ^ een most investi- 

 gated. The living muscle fibre is 

 semi-fluid and translucent. Its fluid 

 nature has been shown by the fact 

 that a nematode worm has been 

 observed to traverse it, and after the 



passage of the invader, the muscle 



Normal I substance to return to its previous 



ordered structure. 



Muscle is very extensile and 

 elastic. The former property is 

 shown by the fact that but a small 

 stretching force is required to change 



Fatigued I * ts sna P e > the latter by the fact that 



when this stretching force is taken 

 off the muscle resumes its previous 

 form. Living muscle has a wide 

 range of this elastic property; it 

 requires a very considerable force to 

 overstep its limits. If a stretching 



FIG. 264. EXTENSIBILITY OF MUSCLE force ** a PP lied suddenly and in- 



IN VARIOUS STATES. (Waller.) creased by equal increments, a living 



Tested by grammes applied muscle extends most at first an. I 



for short periods. then by less amounts till the limit 



of its extension without rupture 



is reached. Conversely, on removing the extending force, the 

 muscle returns at first quickly and then more slowly to its original 

 form. Rubber and most inorganic bodies, such as metal rods, on 

 the other hand, extend almost equally for each increment of the 

 extending force, and return almost equally as the stretching force is 

 removed. Dead muscle is leas extensible and less elastic than living 

 muscle (Fig. 264). 



A contracted muscle is more extensible than a resting one. This 

 gives us the paradox that, if "a muscle were loaded by a weight 



