TONIC CONTRACTION OF MUSCLES. 449 



point, and then ascertained the weight which would balance its power. The 

 same was several times repeated, as in the following manner : The length of 

 the muscle in its extreme state of contraction, at which no additional force 

 could be exerted by it, being represented by 14, it was found that, when it 

 had been extended to 17, it would balance a weight of 60 ; when its length 

 increased to 19'6, it would balance a weight of 120; and at 22'5, it would 

 balance 180. In another experiment, the muscle at 13'5, balanced 0; at IS'S, 

 it balanced 100; and at 23'4, it balanced 200. Hence it appears that a uni- 

 form increase of force corresponds with a nearly uniform increase in the 

 length of the muscle ; or, in other words, that when the muscle is nearly at 

 its full length, its contractile power is the greatest. In later experiments upon 

 the same muscle, this uniform ratio seemed to be departed from ; but, by 

 comparing the results in a considerable number of instances, it was constantly 

 found that, in those experiments which were performed the soonest after the 

 preparation of the frog, and in which, therefore, the normal conditions of the 

 system were the least disturbed, the ratio was very closely maintained. It 

 has been ascertained by Valentin, on repeating these experiments, that, by 

 repeated equal irritations, the strength of the muscles in beheaded frogs de- 

 creases in a regular and corresponding ratio ; losing the same amount in each 

 successive period of time. He also found that, when all the Irritability has 

 ceased, the muscles tear with a far less weight, than they were previously 

 able, when galvanized, to draw. 



a. It has been inferred by Miiller, from Schwann's experiments, that the power, which 

 causes the contraction of a Muscle, must be very different in its character, from any of the 

 forces of attraction known to us ; since these all increase in energy as the attracted parts ap- 

 proach each other, in the inverse ratio of the square of the distance ; so that the power of a 

 Muscle, if operated on by any of these, ought to increase, instead of regularly diminishing, 

 with its degree of contraction. But it is to be remembered that, as the observations of Mr. 

 Bowman have clearly shown, there must be a considerable displacement of the constituents 

 of every fibre during contraction ( 231) ; so that it is easy to understand that, the greater 

 the contraction, the more difficult must any further contraction become. If, between a mag- 

 net and a piece of iron attracted by it, there were interposed a spongy elastic tissue, the iron 

 would cease to approach the magnet at a point, at which the attraction of the magnet would 

 be balanced by the force, needed to compress still further the intermediate substance. 



3. Of Muscular Tonidty. 



593. We have now to consider the other form of Contractility, which pro- 

 duces a constant tendency to contraction (varying, however, as to its degree) 

 in the Muscular fibre ; but which is so far different from simple Elasticity, 

 that it abates after death, before decomposition has taken place. This Toni- 

 city is to be distinguished from the Muscular Tension, which is the result of 

 the reflex operation of the nervous centres ( 398) ; being manifested as well 

 when the muscle is altogether removed from nervous influence, as when sub- 

 jected to it, and being, like Irritability, an inherent property of the tissue itself, 

 the presence of which is characteristic of its living state. It manifests itself 

 in the retraction which takes place in the ends of a living muscle, when it is 

 divided (as is seen in amputation) ; this retraction being permanent, and 

 greater than that of a dead muscle. But its effects are much more remarkable 

 in the non-striated, than in the striated form of Muscular Fibre ; and are par- 

 ticularly evident in the contractile coat of the Arteries, causing the almost 

 entire obliteration of their tubes, when they are no longer distended with 

 blood. The disposition to tonic contraction is increased by any considerable 

 change of temperature ; the power of Heat is well seen in the following ex- 

 periments of John Hunter's : "As soon as the skin could be removed from 

 a sheep that was newly killed, a square piece of muscle was cut off, which 



38* 



