TONICITY OF MUSCLES. RIGOR MORTIS. 213 



dent upon the presence of oxygen in the muscular substance ; conse- 

 quently the irritability departs after death much more speedily in these 

 than in cold-blooded animals. 



364. We have now to consider the other form of Contractility ; which 

 produces a constant tendency to contraction in the Muscular fibre, but 

 which is so far different from simple Elasticity, that it abates after 

 death, before decomposition has taken place. This Tonicity manifests 

 itself in the retraction which takes place in the ends of a living muscle, 

 when it is divided ; the retraction being permanent, and greater than 

 that of a dead muscle. It also shows itself in the permanent flexure 

 of joints, when, by paralysis of the extensors, the tonic contraction of 

 the flexors is not antagonized. In the healthy state, it would seem as 

 if the tonicity of the several groups of muscles was so adjusted, as to be 

 in mutual counterpoise ; but the balance is destroyed, when, in conse- 

 quence of paralysis, or of impaired nutrition from other causes, the' 

 tonicity of one set is weakened. This is the case, for example, in the 

 lead palsy ; in which the extensors of the forearm and hand lose their 

 power, so that the tonic contraction of the flexors keeps the fingers con- 

 stantly bent upon the palm. It would seem, however, that the tonicity 

 of the flexors is usually greater than that of the extensors ; as the 

 former predominate, when all are equally withdrawn from the control of 

 the nervous system, in profound sleep. 



365. The Tonicity is much greater, relatively to the amount of irri- 

 tability, in the non-striated, than in the striated fibre ; and it is parti- 

 cularly remarkable in the fibrous coat of the arteries, in which it is 

 difficult to procure any decided indication- of irritability by the applica- 

 tion of stimuli. It is by this tonicity of the walls of the arteries, that 

 they are kept in a state of constant moderate contraction upon their 

 contents ; and that, when they are emptied, they contract until the 

 tube is nearly obliterated. If its amount be too great (as sometimes 

 happens) the artery approaches the condition of a rigid tube ; which, as 

 will be shown hereafter ( 583), is unfavourable to the regularity of the 

 flow of blood through it, though the rate is increased. On the other 

 hand, if it be unduly diminished, the circulation is retarded, by the 

 tendency of the arterial walls to yield too much to the pulse-wave. 



366. This property is very greatly affected by temperature ; being 

 diminished by warmth and increased by cold. Thus when an artery 

 is exposed to the air for some time, the lowering of its temperature 

 occasions its contraction to such an extent, that its tube may^be almost 

 obliterated. And in the operation of crimping ; fish, immersion of the 

 body in cold water, after the muscles have been divided, increases the 

 tonic contraction of the muscles, and thus improves the firmness of their 

 substance, which it is the object of this operation to produce. 



367. The Rigor Mortis, or death-stiffening of the muscles, is pro- 

 bably to be regarded as a manifestation of this property, occurring 

 after all the irritability of the muscles has departed, but before any 

 putrefactive change has commenced. This phenomenon is rarely ab- 

 sent ; although it may be so slight, and may last for so short a time, as 

 to escape observation. The period which elapses before its commence 

 ment is as variable as its duration ; and both seem to be dependent 



