CHAP, vil.] RHYTHMICAL CONTRACTIONS. 193 



other forms ; but, as we are ignorant of the length of each unstriped 

 fibre, we cannot say whether this slowness is in advance along each 

 one, or merely from one to another. 



The contraction exhibited by the muscles in question is always 

 of the peristaltic character, by whatever stimulus excited ; and its 

 type is therefore probably derived from some peculiarity in the 

 fibres themselves, as in their arrangement. But it is remarkable 

 that the stimuli which usually excite it, are applied in succession to 

 different parts, and are thus entirely suited to the production of the 

 peristaltic contraction. We have a striking example of this in the 

 03sophagus, which is simply a tube of transmission, and not intended 

 to delay the food. The pellet, when thrust into it by the muscles of 

 the pharynx, distends its fibres ; which, then contracting upon it, 

 propel it into a fresh portion ready to receive it. This in its turn 

 contracts, and urges it along ; and so on, until it is conducted to the 

 stomach. 



In this instance, it is evident that the propelled substance is itself 

 the stimulus to the successive contractions. This it may be, either 

 by distending the fibres, and so acting locally upon them ; or else by 

 impressing the nerves cf the membrane touched, in such a way as 

 to excite a nervous stimulus to the muscular coat at each particular 

 part, at the proper moment. As the food is not propelled if the 

 nerves are divided, there can be little doubt that the latter is the 

 true explanation. 



The contraction of the bladder occurs after a gradual distension, 

 and, though very temporary, is probably of the true peristaltic kind. 

 The more protracted action of the uterus is undeniably so. In 

 pregnant animals this may be as distinctly perceived as in the 

 intestines, and it probably occurs during the gradual development 

 of the muscular structure as pregnancy advances; but at length 

 a very powerful impulse occasions the expulsion of the young, and 

 the uterus subsequently remains contracted, because no force dis- 

 tends its fibres. The after-pains mark the final efforts of active 

 contraction. Atrophy of the tissue then occurs, as its develop- 

 ment had done, in accordance with other laws. 



Rhythmical contractions are those which succeed one another 

 after regular intervals of repose. The muscles of respiration and 

 the heart exhibit them through life, which would cease if they were 

 intermitted even for a brief period; for the oxygenation of the 

 blood, and the dispersion of that fluid through the substance of the 

 various organs, must incessantly proceed. Hence neither is an act 

 of the will required for their production, nor could it under any 



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