RIGID DYNAMICS is that portion of theoretical dynamics (q.v.) which,based oti the the ory of the free and constrained motion of points, applies the principles thence deduced to a eptem of points rigidly connected, so as to bear throughout the whole continuance of their motion the same invariable position with relation to each other; in other words, as no body in nature can be considered as a point, but is truly a system of points, rigid dynam ics has for its aim to apply the abstract theory of dynamics to the cases actually occur ring in nature. For a long time problems of this sort were not resolved by any general and adequate method, but each class was worked out according to a method specially applicable to its particular circumstances. The great general principle discovered by the French geometer,commonly known as D'Alembert'sprinciple,which applies equally to all such problems, and removes the necessity for specially investigating each particular case, was an inestimable boon to mechanical science. It is thus stated iu his Traite de Dynamique : " In whatever manner a number of bodies change their motions, if we suppose that the motion which each body would have in the following moment, if it were per fectly free, is decomposed into two others, one of which is the motion which it really takes in consequence of their mutual actions, then the other component will be such, that if each body were impressed by a force which would produce it alone, the whole system would be in equilibrium. In this way every dynamical problem can be compelled to furnish an equation of equilibrium, and so be changed into a problem of statics (q.v.); and thus the solution of a difficult and complex problem is effected by means of the reso lution of a much easier one. D'Alembert applied his principle to various problems on the motions and actions of fluids, the precession of the equinoxes, etc.; and subsequently, in a modified form, the same general property was made the basis of a complete system of dynamics, by La Grange, in his Nicanique Analytique.
MOIt'TIS is the term usually given to the peculiar temporary rigidity of the muscles that occurs shortly after death. It begins immediately after all indications of irritability (see MUSCLE) have ceased, but before the commencement of putrefaction. In the human subject it most commonly begins to show itself about seven hours after death, although cases are occasionally met with in which 20 or even 30 hours may have elapsed before it begins to appear. This condition of rigidity usually lasts for about 30 hours; but it may pas's off in 10 hours or less, or may be prolonged to four or six days. The muscles of the neck and lower jaw are first affected, then those of the trunk, then those of the upper extremities, and lastly those of the lower extremities. In its depar ture, which is immediately followed by decomposition, the same order is followed.
This subject has been admirably discussed by Dr. Brown-Sequard in the " Croonian Lecture" for 1861, and contained in The Proceedings of the Royal Society for that year.
In this lecture he examines successively the relations existing between muscular irritabil ity, post-mortem rigidity, and putrefaction, in a variety of cases. The following are his chief conclusions: 1. Paralyzed muscles are endowed with more irritability than healthy muscles; cadaveric rigidity sets in late,- and lasts long; and putrefaction appears late, and progresses slowly. 2. Experiments' made on numerous animals show that when muscular irritability is increased by a diminution of temperature, the increase has the same effect upon rigidity and putrefaction as when it is caused by paralysis. As a general rule, when there was a difference of 14° to 18° Far. in the temperature of two animals of the same age and species, irritability and rigidity lasted twice or three times longer in the cooler animal than in the other, and putrefaction in the former was much less rapid. 3. It was maintained by John Hunter that cadaveric rigidity does not take place after death by lightning; but it is now known that this view is not generally true. When lightning destroys life by producing such a violent convulsion of every muscle in the body that muscular irritability at once ceases, the ensuing rigidity may be of such short duration as to escape notice; but if it causes death by fright, hemorrhage, or con cussion of the brain, cadaveric rigidity will appear as usual. 4. In animals that have been over-driven, hunted to death, etc., rigidity comes on very quickly, lasts for a very short time, and is rapidly succeeded by putrefaction; and various facts quoted by Brown Sequard show that over-exertion acts similarly in man. 5. The nutrition of the muscles exerts a modifying influence on rigidity and putrefaction. In cases of death from decapitation, strangulation, sudden hemorrhage from a wounded artery, etc., cadaveric rigidity does not begin till 16 or 18 hours after. death, and lasts from six to eight days; while in a case of death from exhaustion. after a prolonged typhoid fever, rigidity became evident within three minutes after the last breathing, while the heart was still beating; disappeared in a quarter of an hour, and was at once succeeded by signs of putrefaction before the man had been dead an hour. 6. When death follows violent and prolonged convulsions (as in cases of tetanus, hydrophobia, etc.), cadaveric rigidity sets in soon (usually within an hour after death), and ceases before the end of the tenth hour; and when the convulsions were caused by strychnine, similar results were obtained.
From these facts. this accomplished physiologist deduces the general law that "the greater the degree of muscular irritability at the time of death, the later the cadaveric rigidity sets in; and the longer it lasts, the later also putrefaction appears, and the slower it progresses." The exact cause of this rigidity is not accurately known. The old view that it depended on the coagulation of the blood is no longer tenable. It most probably results from the spontaneous coagulation of a fibrinous material contained in the muscular juice.