372 THE PROPERTIES OF STRIPED MUSCLE. 



it possible for a muscular fibre to be stimulated into relaxation 

 wakened into sleep. After again stating that the more abrupt and 

 transitory the change of tension, the more marked is its relaxing action, 

 he continues (p. 104) : " A contracted muscle is, according to Pflliger's 

 theory, a labile group of atoms." These he supposes to form a chain, 

 of which the " loosely joined links " are more easily sundered by a 

 " short forcible jerk than by slow persistent tension " ; the latter, on the 

 other hand, is more effectual in evoking contraction, for the very reason 

 that it is itself of relatively slow development. 



The case which seems chiefly to require such a theory is that of the 

 jerk relaxation. Schenck himself tells us that in this experiment the 

 muscle receives an Erschiltterung a shatter a process by which we 

 know experimentally that heat is liberated. It is difficult to accept a 

 doctrine which identifies this with inhibition, and much more easy to 

 suppose that the influence of jerk or shatter is simply to put a 

 .stop to the whole process, by changing the mode in which energy is 

 liberated, substituting the production of heat for that of work, and to 

 regard both the changes which follow excitation relaxation no less 

 than contraction as parts of the same process. We may then inquire 

 whether or not it is necessary to call in the aid of an auxiliary theory of 

 secondary excitation. 1 



The "simple experiment" which led Schenck to the discovery of 

 the remarkable fact, that a muscle which is arrested in the course of an 

 isotonic contraction, if lightly loaded, promptly relaxes, rather suggests 

 questions than answers them. It has been subjected to criticism by 

 other experimenters, particularly by Kaiser, 2 who has shown that both at 

 high and low temperatures, the question whether the muscle relaxes or 

 not immediately after the arrest, depends on the height of the stop, i.e. 

 on the length of the muscle at the time the stop is applied as compared 

 with its natural length when unloaded and unexcited. On this observa- 

 tion he has founded what may be called a new method of observing the 

 phenomena of isotonic contraction and relaxation, which promises to be 

 very fruitful. Starting from Weber's conception of the process, accord- 

 ing to which the excited contractile molecular structure, at the moment 

 of excitation, takes the place of the unexcited, and is replaced by it at 

 the end of the period of excitation (see p. 360), he begins by seeking for a 

 criterion by which the state of excitation may be characterised in a 

 muscle which is allowed to change its form in response to an instan- 

 taneous stimulus without mechanical interference. The character by 

 which an excited muscle can be most easily distinguished from an 

 unexcited one is its length as recorded in the isotonic curve. He 

 therefore takes as the object of observation, a muscle (gastrocnemius) 

 arranged for writing its curve isotonically with the lightest possible load. 



The upper end of the muscle is fixed during the whole course of each 

 series of observations ; consequently the horizontal line which corresponds to 

 the natural length (Fig. 205, I) of the muscle when unexcited and unloaded is 

 also constant. When in the progress of the observation the muscle is loaded, 



1 The paper on this subject already referred to (Arch. f. d. gcs. Physiol., Bonn, Bd. Iv. 

 S. 629) is entitled " Ein einfacher Versuch ztir Demonstration des Einflusses der Spannung, 

 u.s.w." 



2 "Zur Analyse der Zuckungscurve des quergestreiften Muskels," Ztschr. f. iol., 

 Miinchen, 1896, Bd. xxii. S. 157-178; " Untersuch. ueber die Natur der bei der Con- 

 traction des quergestreiften Muskels wirksame Kra'fte," S. 361. 



