CHAP, ii.] THE CONTEACTILE TISSUES. 79 



apparently smooth, continuous effort, is known as tetanus, or 

 tetanic contraction. The above facts are most clearly shewn 

 when induction-shocks, or at least galvanic currents in some 

 form or other, are employed. They are seen, however, what- 

 ever be the form of stimulus employed. Thus, in the case of 

 mechanical stimuli, while a single quick blow may cause a single 

 twitch, a pronounced tetanus may be obtained by rapidly striking 

 successively fresh portions of a nerve. With chemical stimulation, 

 as when a nerve is dipped in acid, it is impossible to secure a 

 momentary application ; hence tetanus, generally irregular in 

 character, is the normal result of this mode of stimulation. In 

 the living body, the contractions of the skeletal muscles, brought 

 about either by the will or otherwise, are generally tetanic in 

 character. Even very short, sharp movements, such as a sudden 

 jerk of a limb, or a wink of the eyelid, are, in reality, examples of 

 tetanus of short duration. 



If the lever, instead of being fastened to the tendon of a muscle 

 hung vertically, be laid across the belly of a muscle placed in a 

 horizontal position, and the muscle be throw.n into tetanus by a 

 repetition of induction-shocks, it will be seen that each shortening 

 of the muscle is accompanied by a corresponding thickening, and 

 that the total shortening of the tetanus is accompanied by a cor- 

 responding total thickening. And, indeed, in tetanus we can 

 observe more easily than in a single contraction that the muscle in 

 contracting 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 prolonged into a narrow glass tube, and the 

 chamber be so filled with water (or, preferably, with a solution of 

 sodium chloride, -6 p. c. in strength, the " normal saline solution," 

 which is less injurious to the tissue than simple water) that 

 the fluid rises up into the narrow tube, it is obvious that any 

 change in the bulk of the muscle will be easily shewn 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 



