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 Ciliw 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 i,vhich 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 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permanent contraction, requiring no stimulus to excite it, shewing itself whenever a dis tending or elongating power is withdrawn, and relaxing only at the close of life.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 (frorn 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 liinbs (which are the stronger) over the extensors during sleepil- and the stiffening or " roideur cadaverique" of the muscles after death,—seem to be clear indica tions of a tendency to contraction answering to the definition of Tonicity, not of Irritability. This last phenomenon, as it disappears before putrefaction begins, and as it is variously in fluenced by causes affecting vital action, is allowed to be a last exertion of vital power.
There are evidently slighter modifications or varieties of the powers which we have thus distinguished ; but the distinctions now stated seem to be those which are sufficiently marked to demand separate names. Besides the mus cular texture, some of the membranes, espe cially the skin, appear to be endowed with a certain degree of vital contractile power, al though not with true Irritability. It is remark able, that the greatest degree of contraction seen in muscular fibres, is in those which pos sess the property of simple Contractility rather than Irritability, viz. in the bladder and uterus more than in the intestines, and in these more than the heart.