720 



Again, the rapidity of the changes of po- 

 sition of the component particles of muscular 

 fibres may be estimated, although it can 

 hardly be conceived, from various well-known 

 facts. The pulsations of the heart can some- 

 times be distinctly numbered in children at 

 more than 200 in the minute; and as each 

 pulsation of the ventricles occupies only one- 

 third of the time from the commencement of 

 one pulsation to the commencement of the 

 next, this implies that each contraction takes 

 place in g igth part of a minute, or that ten 

 times in each second, for many hours toge- 

 ther, the whole of the convoluted muscular 

 fibres of the ventricles must be thrown into 

 folds, and again smoothed out. Again, it is 

 certain that by the movements of the tongue 

 and other organs of speech, 1500 letters can 

 be distinctly pronounced by some persons in 

 a minute. Each of these distinguishable 

 sounds must require a separate contraction of 

 muscular fibres ; and the production and ces- 

 sation of each of these sounds must imply, 

 that each separate contraction must be followed 

 by a relaxation of equal length. Each con- 

 traction must, therefore, have been effected in 

 aouijth part of a minute, or in the 50th part of a 

 second. Haller calculated that in the limbs 

 of a dog at full speed, muscular contractions 

 must take place in less than the 200th part of 

 a second, for at least many minutes in suc- 

 cession.* 



But the property of Irritability, which acts 

 throughout so great a portion of the animal 

 creation, as a moving power of this extra- 

 ordinary efficiency, is not the only contractile 

 power, which certain organic textures possess, 

 or which the conditions of their existence re- 

 quire them to exert during the living state. 

 Even in muscular fibres themselves, in certain 

 organs, and still more in other textures of 

 animal bodies, contractions are often observed, 

 peculiar to the living state, but differing essen- 

 tially from those which come under the defi- 

 nition of Irritability already given. 



In all the different tribes of animals, indeed, 

 differences in the contractile power of the diffe- 

 rent living solids may be observed, exactly cor- 

 responding to their circumstances and wants. 

 The slow and languid movements of the bodies 

 of most of the Zoophyta, and the rapid vibra- 

 tions of the Ciliae with which parts of many 

 of these animals (particularly of the order 

 Infusoria) are provided, are examples, even 

 in the lowest class, of the great variety of 

 moving powers, with which the living solids 

 of different animals are endowed. 



In the human body, and analogous animals, 

 it is obvious that the contractile power exerted 

 by the stomach and intestines in performing 

 their peristaltic movements, although of the 

 same general characters as that of the heart, 

 the contraction of each portion of the tube 

 being followed by a relaxation of that portion 

 and a contraction of the portion next in ad- 

 vance, is yet materially different ; both con- 



* See Haller's Elcm. Phys. torn. iv. p. 481. 



CONTRACTILITY. 



traction and relaxation in the peristaltic move- 

 ment being of longer and less definite du- 

 ration, and of more variable extent. In the 

 bladder and in the uterus, in the healthy state, 

 we see contractions excited by peculiar stimuli, 

 and repeatedly recurring as the actions de- 

 pendent on them proceed, but not alternating 

 with any obvious elongation of the fibres, and 

 terminating in a much greater and more per- 

 manent shortening of the contracting fibres, 

 than is observed in other muscular organs. 



Again, in the state of any voluntary muscle, 

 when the distance of its extremities is per- 

 manently shortened (as by an ill-united frac- 

 ture), in that of the sphincter muscles, or of an 

 artery when emptied of blood, we see a 

 permanent contraction, requiring no stimulus 

 to excite it, shewing itself whenever a dis- 

 tending or elongating power is withdrawn, and 

 relaxing only at the close of life. The nume- 

 rous experiments of Dr. Parry on the condition 

 of arteries immediately after death (contained 

 in his Treatise on the Arterial Pulse) afford the 

 most precise information that we have as to 

 this last property. 



From such facts it appears obvious that 

 three distinct modes of contraction, all strictly 

 vital, may be observed in different textures of 

 the body, or even in the same texture under 

 different circumstances: first, that already con- 

 sidered, to which the term Irritability is strictly 

 applied, and which is best exemplified in the 

 actions of the voluntary muscles and the heart ; 

 secondly, that which may be termed simple 

 Contractility, where contraction is induced by 

 a stimulus, but takes place more slowly, and 

 is nearly or quite permanent; and, thirdly, 

 that which has been accurately described by 

 Dr. Parry and others under the title of Toni- 

 city, which requires no stimulus to call it into 

 action, but takes effect whenever a distending 

 power is withdrawn, and continues until 

 life is extinguished. The second of these 

 forms is seen, not only in the bladder and 

 uterus, but in the arteries under certain irri- 

 tations, perhaps in other textures, and pro- 

 bably also (from certain stimuli) in the fibrin 

 of the blood during coagulation.* The last 

 is clearly, as Dr. Parry's experiments have 

 shewn, the chief vital endowment of arteries; 

 and notwithstanding the doubts expressed on 

 the subject by Dr. Bostock, several facts may 

 be stated to show, that it is also an endow- 

 ment of all muscular fibres. Thus, besides 

 the permanent retraction, already noticed, of 

 the fibres of a muscle the fixed extremities of 

 which are approximated, the retraction of the 

 cut ends of a muscle divided during life, the 

 state of habitual preponderance of the flexor 

 muscles of the body and limbs (which are the 

 stronger) over the extensors during sleep,f and 

 the stiffening or " roideur cadaverique " of the 

 muscles after death, seem to be clear indica- 

 tions of a tendency to contraction answering 



* SeePrater'sExperimental Inquiries iu Chemical 

 Physiology. 



t See Richcrand's Physiology. 



