THE MECHANICAL RESPONSE. 353 



tinuously ; the other, that the response to a single stimulation consists of 

 two processes or stages, in one of which it enters into the state of 

 activity, in the other returns to that of rest. The interference of these 

 two states involves complications which do not present themselves in a 

 muscle which is continuously excited. 



Mechanical response to a succession of instantaneous stimuli 

 Tetanus. A state of excitation comparable to voluntary contraction 

 can be produced for the purpose of investigation, by subjecting a muscle 

 to the action of a succession of instantaneous stimuli of sufficient fre- 

 quency. The response to excitation of this kind is technically called 

 tetanus. 1 In using this term, it should be carefully borne in mind that 

 there are other forms of continuous contraction which are not evoked 

 by repeated stimuli ; and it is by no means certain that voluntary con- 

 traction is not of the number. 



The methods of producing tetanus will be discussed more fully in a 

 subsequent section. For our present purpose it is sufficient to say that 

 the tetanic state (the state of excitatory shortening) can be produced 

 either directly or indirectly, i.e. by subjecting either the muscle itself 

 (direct method) or its nerve (indirect method) to a succession of 

 stimuli. For the purpose now in view it is essential that the stimula- 

 tions should be not only frequent, but of uniform efficiency; and to 

 insure this, they must be of such strength as to make the muscle contract 

 to the utmost. 



The characteristic phenomena of tetanus can be best observed in a muscle 

 which is excited directly by induction currents in alternately opposite direc- 

 tions, of which the frequency is from 30 to 100 per second. It is assumed 

 in what follows that this method is employed. 



For experiments relating to the mechanical response to stimulation, the 

 muscle preparation which has been used by preference is that of the pair of 

 internal thigh muscles of the frog the gracilis and semimembranosus. These 

 are easily recognised by the tendinous inscription with which each is provided. 

 They take their origin from the ischio-pubic symphysis and run nearly parallel 

 to each other, to be inserted into the inner aspect of the head of the tibia- 

 fibula. The preparation is made by cutting through the two heads on each 

 side and removing with each the two muscles in question, together with the 

 ischio-pubic ridge from which they spring. The advantage of this preparation 

 (hereafter referred to as the " double adductor preparation ") is that each of the 

 muscles consists of fibres all of which run in the same direction. It is a 

 further advantage that when prepared as above described the two groups act 

 for all practical purposes as one muscle, the length of which, if they are used 

 lengthwise, may be, in a good-sized frog, as much as 3 in. (Fig. 189). 



The phenomena which present themselves when a muscle is com- 

 pletely and continuously excited, differ according to the mechanical con- 

 ditions under which it is placed. If before and during excitation its 

 opposite attachments are so fixed that they cannot be brought nearer 

 together by the effort of the muscle to contract, the excitation is said 

 to occur under isometric conditions. If, on the other hand, one end of 

 the muscle is left free, so that it can shorten on excitation, and in 



1 The fact that continuous contraction can be produced by repeated stimulation was 

 known to Volta (1792). The word "tetanise" was introduced into physiology by 

 Matteucci (1838). "Tetanus" is now denned in most text-books as the effect of rapidly 

 repeated stimulation (see Hermann's "Handbuch," Bd. i. S. 41). du Bois Reymond, who 

 first used this method for investigating muscular contraction, did not limit the meaning of 

 the word to the result of this kind of stimulation. 

 VOL. II. 23 



