450 OF MUSCULAR CONTRACTION. 



was afterwards divided into three pieces, in the direction of the fibres ; each 

 piece was put into a basin of water, the water in each basin being of different 

 temperatures, viz., one about 125, about 27 warmer than the animal; an- 

 other 98, the heat of the animal; and the third 55, about 43 colder than 

 the animal. The muscle in the water heated to 125 contracted directly, so 

 as to be half an inch shorter than the other two, and was hard and stiff. The 

 muscle in the water heated to 98 after six minutes began to contract and 

 grow stiff; and at the end of twenty minutes it was nearly, though not quite, 

 as short and hard as the above. The muscle in the water heated to 55 after 

 fifteen minutes, began to shorten and grow hard; after twenty minutes it was 

 nearly as short and as hard as that in the water heated to 98. At the end 

 of twenty-four hours, they were all found to be of the same length and stiff- 

 ness."* The agency of Heat in producing this contraction is also remarkably 

 shown in the fact, that if a Frog be immersed in water of the temperature of 

 110, the muscles of its body and limbs will be thrown into a state of perma- 

 nent and rigid contraction. But it would seem that these effects are chiefly, 

 if not entirely, exerted upon the striated form of Muscular fibre ; and that the 

 tonicity of the non-striated fibre is called into play by Cold, rather than by 

 heat. For if a Tadpole or Frog be immersed in water, the temperature of 

 which is gradually raised, until this state of contraction comes on, the Heart 

 will be found to continue pulsating for many hours afterwards, not being 

 affected by the heat. On the other hand, if an artery in a living warm-blooded 

 animal be exposed to cold air for some time, the lowering of its temperature 

 occasions its contraction to such an extent, that its cavity becomes almost 

 obliterated. The influence of warmth in diminishing, and of cold in increas- 

 ing, the tonicity of the arterial system, will be adverted to hereafter (Chap. 

 xii., Sect. 3). 



594. The distinctness of the Tonicity of Muscles from their Irritability, is 

 further shown by the fact that the former commonly survives the latter ; and 

 that it is not destroyed by treatment, which occasions the complete departure 

 of the Irritability. The first of these statements finds its proof in the pheno- 

 mena of the Rigor Mortis, presently to be adverted to. Of the latter, the fol- 

 lowing remarkable experiment of John Hunter's is an ample demonstration: 

 " From a straight muscle in a bullock's neck, a portion, three inches in length, 

 was taken out immediately after the animal had been knocked down, and was 

 exposed between two pieces of lead, to a cold below 0, for fourteen minutes ; 

 at the end of this time it was found to be frozen exceedingly hard, was be- 

 come white, and was now only two inches long; it was thawed gradually, and 

 in about six hours after thawing, it contracted so as only to measure one inch 

 in length ; but irritation did not produce any sensible motion in the fibres. 

 Here, then, were the juices of muscles frozen, so as to prevent all power of 

 contraction in their fibres, without destroying their life ; for when thawed, 

 they showed the same life which they had before ; this is exactly similar to 

 the freezing of blood too fast for its coagulation, which, when thawed, does 

 afterwards coagulate, as it depends in each on the life of the part not being 

 destroyed."! 



595. The Rigor Mortis, or death-stiffening of the muscles, is probably to 

 be regarded as the final manifestation of tins property ; occurring after all the 

 Irritability of the muscles has departed, but before any putrefactive change 

 has commenced. This phenomenon is rarely absent ; though it may be so 

 slight, and may last for so short a time, as to escape observation. The period 

 which elapses before its commencement, is as variable as its duration ; and 



* General Principles of the Blood, in Hunter's Works, Vo). iii., p. 110. . 

 t Op. cit., p. 109. 



