88 niYSioLOGY OF ml'sci.p:s and nerves. 



stiffening intervenes mucli more quickly in muscles 

 which have been strongly irritated before death, as for 

 instance in those of hunted animals. But while the 

 formation of acid must always be very slight in active 

 muscle, it increases greatly in muscles which have un- 

 dergone death-stiffening, and the acid acts as a relax- 

 ing agent on the connective tissue which holds the 

 fibres together, so that the latter separate more readily. 

 At the same time, however, another distinct change 

 occurs within the muscle-fibre. If a fresh living muscle- 

 fibre and one that has undergone death-stiffening are 

 examined under the microscope, the latter appears dull 

 and opaque ; the transverse striations are narrower and 

 approach more nearly together, and the contents are 

 not active and fluid, as in the living fibre, but are fixed 

 and broken into fragments. When unextended muscles 

 undergo death-stiffening, they usually become shorter 

 and thicker. In the mobile facial muscles of a dead 

 body the result of this is that the lines, which imme- 

 diately after death were relaxed, again acquire a certain 

 expression. The death-stiffening of the muscles is the 

 cause of a certain rigidity in the limbs of corpses, so 

 that the limbs are retained in the same relative posi- 

 tion in which they were at death ; and it is to this 

 circumstance that the name ' death-stiffening ' [rigor 

 mortis) is principally due. Moreover, this change docs 

 not occur simultaneously in the muscles of all parts of 

 the dead body ; it usually begins in the muscles of the 

 face and neck and passes gradually downward, so that 

 the muscles of the legs are the last to be affected by 

 it. The relaxation of the rigidity takes place in the 

 same order. 



On account of the shortening undergone by muscles 



