MUSCULAR TONUS. 377 



rises to its maximal height, after which it remains for a time at the same 

 level — indicating that the muscle is fully tetanised. In this state it is 

 free from stress of any kind, so that its actual length and equilibrium 

 length, which may be designated A, are the same. Although we have 

 no experimental data which enable us to state what the relation of a 

 to A is, there is reason to believe that the former length exceeds the 

 latter by about three-quarters. In a muscle excited in the manner 

 above described, the tetanic state is produced not so much by the super- 

 position of effects (as in Helmholtz's well-known experiments), as by the 

 summation of stimuli ; for each excitation occurs before the contraction 

 evoked by its predecessor has begun. Consequently, in the transition of 

 a muscle from the imperfect state of excitation to which a relates, to 

 that of complete tetanus, there may be any number of stages of 

 contraction, each characterised by an equilibrium length intermediate 

 between X and A. In this connection it will be remembered that in the 

 response to two maximal stimuli, following each other at a very short 

 interval, the equilibrium length a 2 takes a higher position in the curve 

 than in that which follows a single excitation of the same strength 

 (Fig. 209). 



Muscular tonus. — Throughout the preceding discussions relating 

 to the mechanical properties of muscle, it has been found convenient 

 to regard the length of an unexcited muscle as constant, and to designate 

 it by the letter /. There are, however, many instances which show that 

 a muscle may, although at rest and under conditions which cannot be 

 regarded as abnormal, be of a different length at different times. States 

 of relaxation, in which muscles are longer than they are ordinarily, are 

 not recognised ; so that we need not consider them. It is only necessary 

 to designate those conditions in which the fibres are permanently short. 

 All such states come under the term Tonus. 



The lengthening which the muscles of the higher animals undergo, 

 in consequence of destruction of the motor centres by which they are 

 innervated, may be taken as evidence that in the natural state their 

 fibres are not perfectly at rest. This state, to which the term " skeletal 

 tone " 1 is applied, is essentially one of continuous indirect excitation ; for 

 it manifestly depends on the influence of the nervous system. There 

 are, however, conditions of tone in the production and maintenance of 

 which the central nervous system appears to take no part. Of these 

 the most instructive examples are (1) the tone of the claw muscles of 

 the crayfish ; and (2) that of the heart of Helix. 



The first of these has been most recently investigated by Piotrowsky, 

 who has shown that lengthening of the abductor muscle can be pro- 

 duced as easily by direct as by indirect stimulation, provided that 

 the muscle when stimulated is in a tonic state. The phenomenon 

 can, however, be only observed in winter, and then only when the 

 temperature is about 8° C. or lower. At higher temperatures the 

 muscle contracts normally when stimulated, so that, according to the 

 degree of tonus present, you may have contraction, relaxation, or a 

 mixed result. 



The heart of the snail affords quite as striking an example of the 

 same relation between excitatory relaxation and tonus. The mode of 

 observing it is as follows : — A narrow glass tube is drawn out at one 

 end, and shaped into a cannula, to which the ventricle is tied. The 



1 Foster's " Physiology," 7th edition, part 3, p. 996. 



