CHAP, ii.] THE CONTKACTILE TISSUES. 75 



different animals, in different muscles of the same animal, and in 

 the same muscle under' different conditions. 



The muscle-curve which we have been discussing is a curve of 

 changes in the length only of the muscle ; but if the muscle, instead 

 of being suspended, were laid flat on a glass plate, and a lever laid 

 over its belly, we should find, upon sending an induction-shock 

 into the nerve, that the lever was raised, shewing that the muscle 

 during the contraction became thicker. And if we took a graphic 

 record of the movements of the lever, we should obtain a curve 

 very similar to the one just discussed ; after a latent period the 

 lever would rise, shewing that the muscle was getting thicker, and 

 afterwards would fall, shewing that the muscle was becoming thin 

 again. In other words, in contraction the lessening of the muscle 

 lengthwise is accompanied by an increase crosswise ; indeed, as we 

 shall see later on, the muscle in contracting is not diminished in 

 bulk at all (or only to an exceedingly small extent, about T Q^-Q -g- of 

 its total bulk), but makes up for its diminution in length by 

 increasing in its other diameters. 



48. A single induction-shock is, as we have said, the most 

 convenient form of stimulus for producing a simple muscular con- 

 traction, but this may also be obtained by other stimuli, provided 

 that these are sufficiently sudden and short in their action, as, for 

 instance, by a prick of, or sharp blow on, the nerve or muscle. For 

 the production of a single, simple muscular contraction, the changes 

 in the nerve leading to the muscle must be of such a kind as to 

 constitute what may be called a single nervous impulse, and any 

 stimulus which will evoke a single nervous impulse only may be 

 used to produce a simple muscular contraction. 



As a rule, however, most stimuli other than single induction- 

 shocks tend to produce in a nerve several nervous impulses, and, 

 as we shall see, the nervous impulses which issue from the central 

 nervous system, and so pass along nerves to muscles, are, as a rule, 

 not single and simple, but complex. Hence, as a matter of fact, 

 a simple muscular contraction is within the living body a com- 

 paratively rare event (at least as far as the skeletal muscles are 

 concerned,) and cannot easily be produced outside the body other- 

 wise than by a single induction-shock. The ordinary form of 

 muscular contraction is not a simple muscular contraction, but the 

 more complex form known as a tetanic contraction, to the study 

 of which we must now turn. 



Tetanic Contractions. 



49. If a single induction-shock be followed at a certain 

 interval by a second shock of the same strength, the first simple 

 contraction will be followed by a second simple contraction, both 



