76 THE CONTRACTILE TISSUES. 



ing changes in form only not in bulk. If a living muscle or group of 

 muscles be placed in a glass jar or chamber, the closed top of which is pro- 

 longed into a narrow glass tube, and the chamber be filled with water (or 

 preferably with a solution of sodium chloride, 0.6 per cent, in strength, 

 usually called " normal saline solution," which is less injurious to the tissue 

 than simple water) until the water rises into the narrow tube, it is obvious 

 that any change in the bulk of the muscle will be easily shown by a rising 

 or falling of the column of fluid in the narrow tube. It is found that when 

 the muscle is made to contract, even in the most forcible manner, the change 

 of level in the height of the column which can be observed is practically 

 insignificant ; there appears to be a fall indicating a diminution of bulk to 

 the extent of about one ten-thousandth of the total bulk of the muscle. So that 

 we may fairly say that in a tetanus, and hence in a simple "contraction, the 

 lessening of the length of the muscle causes a corresponding increase in the 

 other directions ; the substance of the muscle is displaced, not diminished. 



50. So far we have spoken simply of an induction-shock or of induction- 

 shocks without any reference to their strength, and of a living or irritable 

 muscle without any reference to the degree or extent of its irritability. But 

 induction-shocks may vary in strength, and the irritability of the muscle 

 may vary. 



If we slide the secondary coil a long way from the primary coil, and thus 

 make use of extremely feeble induction-shocks, we shall probably find that 

 these shocks, applied even to a quite fresh muscle-nerve preparation, produce 

 no contraction. If we then gradually slide the secondary coil nearer and 

 nearer the primary coil, and keep on trying the effects of the shocks, we shall 

 find that after a while, in a certain position of the coils, a very feeble con- 

 traction makes its appearance. As the secondary coil comes still nearer to 

 the primary coil, the contractions grow greater and greater. After a while, 

 however and that, indeed, in ordinary circumstances, very speedily increas- 

 ing the strength of the shock no longer increases the height of the contrac- 

 tion ; the maximum contraction of which the muscle is capable with such 

 shocks, however strong, has been reached. 



If we use a tetanizing or interrupted current, we shall obtain the same 

 general results ; we may, according to the strength of the current, get no 

 contraction at all, or contractions of various extent up to a maximum, which 

 cannot be exceeded. Under favorable conditions the maximum contraction 

 may be very considerable ; the shortening in tetanus may amount to three- 

 fifths of the total length of the muscle. 



The amount of contraction, then, depends on the strength of the stimulus, 

 whatever be the stimulus ; but this holds good within certain limits only ; 

 to this point, however, we shall return later on. 



51. If, having ascertained in a perfectly fresh muscle-nerve prepara- 

 tion the amount of contraction produced by this and that strength of stim- 

 ulus, we leave the preparation by itself for some time say for a few hours 

 and then repeat the observations, we shall find that stronger stimuli- 

 stronger shocks, for instance are required to produce the same amount of 

 contraction as before; that is to say. the irritability of the preparation, the 

 power to respond to stimuli, has in the meanwhile diminished. After a 

 further interval we should find the irritability still further diminished ; even 

 very strong shocks would be unable to evoke contractions as large as those 

 previously caused by weak shocks. At last we should find that no shocks, 

 no stimuli, however strong, were able to produce any visible contraction what- 

 ever. The amount of contraction, in fact, evoked by a stimulus depends not 

 only on the strength of the stimulus, but also on the degree of irritability of 

 the muscle-nerve preparation. 



