THE PROPERTIES OF MUSCULAR TISSUE. 133 



until the summit of the contraction is reached at w. The 

 muscle does not now instantly relax, but only gradually passes 

 back to the resting state : beginning at w, we see the descent 

 of the curve is for a time slow, then more rapid, and finally 

 slow again from x to y, when the contraction is completed 

 and the lever once more traces only the horizontal line yp, due 

 to the continued movement of the recording surface. The 

 curve then shows three distinct phases in the contraction : the 

 period of latent excitation; the period of shortening; the 

 period of elongating. Knowing the rate of horizontal move- 

 ment, we can measure off the time occupied by each phase. 

 The horizontal distance from I to u represents the time taken 

 by the latent excitation 5 from u to z, the time occupied in 

 shortening; from z to y, the time taken in elongation: in a 

 fresh frog's muscle these times are respectively T f^, T ^, T f 

 of a second. In the muscles of warm-blooded animals they 

 are all shorter, but the difficulties in the way of accurate ex- 

 periment are very great. If we know the relative lengths of 

 the arms of the lever we can of course readily calculate from 

 the height, wz, of the curve the extent of shortening of 

 the muscle. With a single electrical stimulation this is never 

 more than one fourth the total length of the muscle. 



In Fig. 62 the accessory apparatus used in practice to in- 

 dicate on the moving surface the exact instant of stimulation 

 and to measure the rate at which S moves have been omitted. 



Physiological Tetanus. It is obvious that the ordinary 

 movements of the Body are not brought about by such tran- 

 sient muscular contractions as those just described. Even a 

 wink lasts longer than one tenth of a second. Our movements 

 are, in fact, due to more prolonged contractions which may be 

 described as consisting of several simple contractions fused 

 together, and known as " tetanic contractions "; it might be 

 better to call them "compound contractions," since the word 

 tetanus has long been used by pathologists to signify a dis- 

 eased stato, such as occurs in strychnine poisoning and hydro- 

 phobia, in which most of the muscles of the Bcdy are thrown 

 into prolonged and powerful involuntary contractions. 



If, while a frog's muscle is still shortening under the in- 

 fluence of one electric shock, another stimulus be given it, it 

 will contract again and the new contraction will be added on 

 to that already existing, without any period of elongation 

 occurring between them. While the muscle is still contract- 



