MOTION 



43 



more specific, because it develops the pressure which is required to 

 drive the blood through the circulatory channels. 



The principle of action, however, is the same in all three cases, 

 because every muscular movement consists of two phases, namely, a 

 period of contraction and a period of relaxation. During the former 

 stage the individual cells or fibers shorten and thicken, while during 

 the latter they assume their original long and thin shape. Obviously, 

 if each constituent undergoes these changes, the muscle as a whole 

 must present very similar alterations. Its contraction is characterized 

 by a decrease in its length in favor of its breadth, and its relaxation, 

 by a decrease in its breadth in favor of its length. During the first 

 period, therefore, its outhne is more spherical. 



Fig. 12. — Stentor Cceruleus, Show- 

 ing Myoids. 

 A, position of rest; J?, contracted state 

 upon stimulation. 



Fig. 13. — Vorticella. 

 A, resting position; B, 

 contracted upon stimu- 

 lation. 



The Structure of Muscle Tissue.^ — The chief element of muscle 

 tissue is the muscle cell which, in the case of the striated type, is gen- 

 erally designated as a fiber. The latter term seems the more appro- 

 priate, because they may attain a length of 30 to 40 mm. or, as some 

 authors claim, of 100 to 150 mm. Their thickness varies between 0.1 

 to 0.01 mm., differing not only in different muscles, but also in the 

 same muscle. Their thickness, in particular, may be much increased 

 by exercise and also during certain pathological conditions, such as 

 hypertrophy and dystrophia muscularis. If we confine ourselves to the 

 striated type, constituting the mass of the skeletal musculature, we 

 find that each muscle is invested by a connective-tissue sheath (peri- 

 mysium) which then extends into its interior (epimysium) and forms 

 small compartments in which the individual muscle fibers are con- 



1 Kollicker's Gewebelehre, Leipzig, 1889, and Schafer, Essentials of Histology, 

 London, 1916. 



