134 



SPECIAL PHYSIOLOGY, 



excessive heat, or be exposed to extreme cold, they will be in the 

 former case exhausted, and in the latter benumbed. If the artery 

 supplying a muscle, or set of muscles, be tied, their contractile power 

 is destroyed ; and if the blood be venous, or charged with carbonic 

 acid, it will impair or destroy their irritability. 



When a muscle ceases to act, it relaxes, or again elongates more or 

 less, according to the position of the bones to which its ends are at- 

 tached ; and muscles evidently possess a certain amount of flexibility 

 and elasticity, or resilient power, to adapt them to the changing posi- 

 tions of the limbs at the joints, and to the various conditions of length 

 rendered necessary by those changes, even when the muscular fibres 

 are in a state of inaction. The elasticity of muscular tissue is, how- 

 ever, very slight, and it diminishes during contraction. It would seem 

 to be much greater, but much less perfect in its action, in the dead 

 than in the living muscle. Thus, a dead muscle requires a greater 

 force to stretch it, but, unlike a living muscle, does not return to its 

 original length when the force is removed. A living portion of muscle 

 undergoes an extension or elongation, when a certain weight is ap- 

 pended to it ; the amount of elongation with moderate weights, is pro- 

 portioned to the weight, but, with greater weights, the effect is no 

 longer proportional ; in dead muscle, and also in paralyzed muscles, 

 the relative elongation is less. The physical cohesive power, or abso- 

 lute strength of muscular tissue, increases up to the adult condition, 

 and then diminishes. It is said to be greater during the so-called 

 rigor mortis, but it decreases some time after death, when the muscles 

 tear more easily. 



After a muscle has contracted a certain number of times, a sense of 

 fatigue or exhaustion is experienced in it, a sensation which must be 

 transmitted to the sensorium through the special sensory nerves of 

 the muscle. It is these nerves also which must convey to the mind 

 accurate information concerning the condition of the muscle, and the 

 amount of effort which it puts forth in any particular action. It is 

 also by these nerves that the impressions which cause the sense of pain 

 in cramps, or other morbid conditions of muscle, are conveyed to the 

 brain. That kind of sensation, which informs us of the amount of 

 action in a muscle, is called the muscular sense ; it is by this that we 

 judge of different weights, and are able to maintain continued muscular 

 effort. The other muscular sensations are probably only modifications 

 of this sense. 



When a muscle is quite fatigued, it requires rest or repose for the 

 recovery of its exhausted irritability. Excessive exercise of a mus- 

 cle, with due intermediate intervals of rest, increases, not only its 

 contractile power and facility of action, but also tends sooner or later 

 to an over-nutrition and increased development of its bulk, or hyper- 

 trophy, probably, as is supposed, from an increase in the size of its 

 individual fibres, and not by the addition of new ones. If, on the 

 other hand, a muscle be not sufficiently exercised, it falls in a state of 

 atrophy, or wasting, or even undergoes a fatty change in its fibres, the 

 striae of which disappear; in either case, its contractile force is di- 

 minished and ultimately lost. The same changes and loss of irrita- 



