THE PROPERTIES OF MUSCULAR TISSUE 93 



of elongating. Knowing the rate of horizontal movement, we can 

 measure off the time occupied by each phase. The horizontal 

 distance from t to u represents the time taken by the latent period; 

 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 re- 

 spectively 1^5, j^, -^ of a second. In the muscles of warm-blooded 

 animals they are all shorter, but the difficulties in the way of 

 accurate experiment 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. 50 the accessory apparatus used in practice to indicate 

 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 move- 

 ments of the Body are not brought about by such transient mus- 

 cular 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 diseased state, such as occurs in 

 strychnine poisoning and hydrophobia, in which most of the 

 muscles of the Body are thrqwn into prolonged and powerful in- 

 voluntary contractions. 



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

 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 contracting under the influence of 

 the second stimulus a third electric shock will make it contract 

 more, and so on, until the muscle is shortened as much as is possi- 

 ble to it for that strength of stimulus. If now the stimuli be re- 

 peated at the proper intervals, each new one will not produce any 

 further shortening, but, each acting on the muscle before the 

 effect of the last has begun to pass off, the muscle will be kept in a 

 state of permanent or "tetanic" contraction; and this can be 



