676 A MANUAL OF PHYSIOLOGY 



is usually from above downwards, beginning at the jaws and 

 neck, then reaching the arms, and finally the legs. After two 

 or three days the rigor disappears in the same order. The 

 position of the limbs in rigor is the same as at death ; the muscles 

 stiffen without any marked contraction. This can be strikingly 

 shown on a newly-killed animal by cutting the tendons of the 

 extensors of one foot and the flexors of the other ; when natural 

 rigor comes on, the feet remain just as they were. If heat 

 rigor, however, is caused, the one foot becomes rigid in flexion 

 and the other in extension ; and the contraction-force is con- 

 siderable, although not so great as that of an electrical tetanus 

 in a living mucsle. 



The Removability of Rigor. After interrupting the circula- 

 tion in the hind-legs of rabbits by compression or ligation of 

 the abdominal aorta (Stenson's experiment), and so causing 

 the muscles to become rigid, Brown-Sequard saw them recover 

 their irritability when the blood was again allowed to reach them. 

 He performed a similar experiment with artificial circulation 

 through the hand of an executed criminal, with a like result. 

 But most writers have taken the view that rigor is the irrevocable 

 end of excitability, and that the apparent recovery which 

 Brown-Sequard saw was due to the muscles not having been 

 completely rigid. Heubel has, however, stated that rhythmical 

 contractions of the frog's heart can be restored by filling its 

 cavity with blood, after rigor has been caused by heat and in 

 other ways, and we have already seen that the same is true of 

 the mammalian heart after the onset of rigor. Excised frog's 

 muscles which have undergone rigor mortis become less stiff 

 when exposed to an atmosphere of oxygen. Both mammalian 

 and frog's skeletal muscles, after rigor mortis has come on, are 

 said to regain their excitability in physiological salt solution 

 (Mangold). 



