560 THE TIME RELATIONS OF MUSCULAR CONTRACTION. 



fractive layer less so. As a result, the fiber may with a certain degree 

 of shortening (2) appear homogeneous and only faintly striated when 

 observed in ordinary light, the homogeneous or transitional stage (Mer- 

 kel's stage of dissolution). If the shortening be more pronounced (3), 

 distinct dark striae again appear, corresponding to the singly refractive 

 discs. At every stage of shortening, including, therefore, the transition- 

 stage, the singly and doubly refractive layers may be demonstrated, by 

 means of the polarizing apparatus, as sharply defined, regularly alternat- 

 ing layers (in i, 2, 3, to the right). They do not exchange places in the 

 muscle-compartment during contraction. The height of both layers is 

 diminished during contraction, that of the singly refractive much more 

 rapidly than that of the doubly refractive layer. The total volume of 

 each element is not appreciably changed during contraction. There- 

 fore, the doubly refractive layers increase in volume at the expense of the 

 singly refractive layers. Hence it follows that during contraction fluid 

 passes from the singly into the doubly refractive layer; the former 

 shrinks, the latter swells. 



Method. The phenomena described can be best observed by instantaneously 

 coagulating the living muscle-fibrils of insects in the various stages of rest or 

 contraction by suddenly applying alcohol or dilute perosmic acid to the muscles, 

 and thus fixing the different stages. The movement itself may be followed under 

 the microscope, either by stimulating the thin, outspread muscle electrically, or, 

 still better, by observing the independent muscular contractions in the trans- 

 parent parts of an insect, for example in the head of the gnat's larva. 



A thin, extended muscle, for example the sartorius of the frog, yields a double 

 spectrum (like a Nobert's glass screen), if light be allowed to pass through a 

 narrow slit, held closely in front of the fibers and at right angles to them. If the 

 muscle be made to contract, for example by mechanical stimulation, the spectrum 

 broadens, an evidence that the intervals between the transverse stria? become 

 smaller. At the same time the transparency of the muscle becomes greater than 

 during rest. 



THE TIME-RELATIONS OF MUSCULAR CONTRACTION. 



MYOGRAPHY. SIMPLE CONTRACTION. TETANUS. 



ISOTONY. ISOMETRY. 



Isotonic muscular activity is the term applied to the contraction in 

 which the tension of the muscle remains the same, while the fibers be- 

 come shorter. 



Method. The time-relations of the contraction in the isotonic muscular act 

 may be shown by v. Helmholtz's myograph (Fig. 192). The suspended muscle 

 (M), fastened at its upper extremity (K), is attached by its lower extremity to 

 a lever constructed like a balance, which can be weighted by means of the 

 weights (W) as desired, and is raised by the shortening of the muscle. From the 

 free extremity of the arm of the lever is suspended by means of a hinge- joint 

 a style (F), which records the movement of the lower extremity of the muscle 

 on the smoked surface of a cylinder made by means of clockwork to rotate at a 

 uniform speed in front of the style. In this way the contracting muscle itself 

 records its contraction-curve, in which the abscissas represent the units of time 

 calculated from the known rapidity of rotation of the cylinder, and the ordinates 

 represent the degree of shortening at any particular moment. 



Fick improved the myograph materially by making the writing lever ex- 

 ceedingly light, and applying the weight close to the rotation-axis of the balance. 

 In this way the swinging movement accompanying the muscular contraction is 

 reduced to a minimum, as is also the change in tension brought about by such 

 movements. 



The surface intended for the reception of the myogram must be moved rapidly, 

 as the process of movement takes place rapidly. Therefore, either a plate fastened 



