CAUSES AND PHENOMENA OF MOTION. 37 



they excite, if any movement, only one that ensues slowly, is compara- 

 tively slight, alternates with rest, and continues for a time after the 

 stimulus is withdrawn. 



In their mode of responding to these stimuli, all the skeletal muscles, 

 or those with transverse striae, are alike; but among those with plain or 

 unstriped fibres there are many differences, a fact which tends to con- 

 firm the opinion that their peculiarity depends as well on their connection 

 with nerves and ganglia as on their own properties. The ureters and 

 gall-bladder are the parts least excited by stimuli: they do not act at all 

 till the stimulus has been long applied, and then contract feebly, and to a 

 small extent. The contractions of the caecum and stomach are quicker 

 and wider-spread: still quicker those of the iris, and of the urinary blad- 

 der if it be not too full. The actions of the small and large intestines, of 

 the vas deferens, and pregnant uterus, are yet more" vivid, more regular, 

 and more sustained; and they require no more stimulus than that of the 

 air to excite them. The heart, on account, doubtless, of its striated 

 muscle, is the quickest and most vigorous of all the muscles of organic 

 life in contracting upon irritation, and appears in this, as in nearly all 

 other respects, to be the connecting member of the two classes of muscles. 



All the muscles retain their property of contracting under the influence 

 of stimuli applied to them or to their nerves for some time after death, 

 the period being longer in cold-blooded than in warm-blooded Verte- 

 brata, and shorter in Birds than in Mammalia. It would seem as if the 

 more active the respiratory process in the living animal, the shorter is the 

 time of duration of the irritability in the muscles after death; and this 

 is confirmed by the comparison of different species in the same order of 

 Yertebrata. But the period during which this irritability lasts, is not the 

 same in all persons, nor in all the muscles of the same persons. In a 

 man it ceases, according to Nysten, in the following order: first in the 

 left ventricle, then in the intestines and stomach, the urinary bladder, 

 right ventricle, oesophagus, iris; then in the voluntary muscles of the 

 trunk, lower and upper extremities; lastly in the right and left auricle of 

 the heart. 



III. EIGOE MORTIS. 



After the muscles of the dead body have lost their irritability or capa- 

 bility of being excited to contraction by the application of a stimulus, 

 they spontaneously pass into a state of contraction, apparently identical 

 with that which ensues during life. It affects all the muscles of the 

 body; and, where external circumstances do not prevent it, commonly 

 fixes the limbs in that which is their natural posture of equilibrium or 

 rest. Hence, and from the simultaneous contraction of all the muscles 

 of the trunk, is produced a general stiffening of the body, constituting the 

 rigor mortis or post-mortem rigidity. 



