292 PHENOMENA OF MUSCULAR MOTION. 



patella have been repeatedly broken by the mere power of the 

 muscles, inserted into them. 



The rapidity with which the successive contractions of the 

 same parts take place is extreme ; and as a striking proof of it, 

 the motions of the tongue, in rapid speaking or reading, are 

 referred to by physiologists. 



The extent or degree of mustular contraction, is in some cases, 

 very great. In proof of this it was stated by the second 

 Monro, that crude mercury, which passes so readily through the 

 intestines, could not be carried along any parts of them whose 

 position happened to be perpendicular, (as the colon on the right 

 side when we stand,) unless the circular fibres of the intestine 

 contract behind it to such a degree as to close completely the 

 cavity of that tube. 



An interesting question may be proposed here, Whether 

 the power of motion, as above described, is exclusively enjoyed 

 by muscular fibres; whether these fibres must be supposed to 

 exist in all those parts of the body which occasionally perform 

 contraction ? 



It has often been inferred, that parts were muscular because 

 they were capable of contraction ; but the question above ought 

 to be decided affirmatively before such inferences can be pro- 

 perly made. 



The sac of the taenia hydatigena appears to be a membrane 

 of a peculiar structure, very different from muscle ; yet it is as 

 capable of contracting as if it were perfectly muscular.* 



The membrane of the urethra does not appear to be muscular 

 in its structure ; and yet it has been seen to protrude a bougie, 

 which had passed near to the neck of the bladder, in a way that 

 indicated regular successive contraction, throughout its whole 

 extent. 



The question above stated, may, therefore, be considered as 

 not yet decided affirmatively. 



Muscular fibres are situated very differently in different parts. 

 They compose almost the whole substance of the heart, which 



* See the Croonian Lecture by Mr. Home ; London Philosophical Transac- 

 tions for 1795, Part I. page 204. 



