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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



ending of the nerves, and in addition describes in the 

 muscularis mucosae of the intestine a large globular 

 end-organ which he considers as a motor plate. 



Perhaps the most striking physiological peculiarity 

 of plain muscle, as compared with the more familiar 

 striated muscle, is the sluggishness of its contrac- 

 tions. Plain muscle, like striated muscle, is inde- 

 pendently irritable. Various forms of artificial 

 stimuli, such as electrical currents, mechanical, 

 chemical, and thermal stimuli, may cause the tis- 

 sue to contract when directly applied to it, but the 

 contraction in all cases is characterized by the 

 slowness with which it develops. There is a long 

 latent period, a gradual shortening which may per- 

 sist for some time after the stimulus ceases to act, 

 and a slow relaxation. These features are repre- 

 sented in the curve shown in Figui'e 68, which it is 

 instructive to compare with the typical curve of a 

 striated muscle (Vol. II.). The slowness of the con- 

 traction of plain muscle seems to depend upon the 

 absence of cross striation. Striped muscle as found in 

 various animals or in different muscles of the same 

 animal — e. g. the pale and red muscles of the rabbit 

 — differs greatly in the rapidity of its contraction, 

 and it has been shown that the more perfect the cross 

 striation the more rapid is the contraction. The 

 cross striation, in other words, is the expression of a 

 mechanism or structure adapted to quick contractions 

 and relaxations, and the relatively great slowness of 

 movement in the plain muscle seems to result from 

 the absence of this particular structure. It should be 

 added, however, that plain muscle in different parts 

 of the body exhibits considerable variation in the 

 rapidity with which it contracts under stimulation, 

 the ciliary muscle of the eyeball, for example, being 

 able to react more rapidly than the muscles of the in- 

 testines. The gentle prolonged contraction of the plain 

 muscle is admirably adapted to its function in the 

 intestine of moving the food-contents along the canal 

 with sufficient slowness to permit normal digestion 

 and absorption. Like the striated muscle, and un- 

 like the cardiac muscle, plain muscle is capable of 



I ;<.. 68.— Contraction of a stri]> of plain muscle from tin- stomach of a terrapin. The bottom line 

 gives the time-record in seconds ; tin- middle line shows the time of application of the stimulus, a tetan- 

 izing current from an induction coil ; the upper line is the curve recorded by the contracting muscle. 



