440 



THE PROPERTIES OF STRIPED MUSCLE. 



phenomenon of the systole of this organ is the apparent diminution of its 

 volume. As in the case of skeletal muscle, this diminution is only 

 apparent. Each of the individual muscular fibres of which the organ is 

 made up, occupies approximately the same space in systole as it did in 

 diastole ; it is its shape that has altered. All the fibres have widened 

 and shortened, and, in consequence, the blood which previously occupied 

 the central cavity and the innumerable channels which, in the frog's 

 ventricle, extend therefrom almost to the surface, has been squeezed out. 

 If the expulsion of the blood takes place freely > each fibre assumes its 

 new form with very inconsiderable production of tension. If it is so 

 resisted that no blood can escape, a state of effort comes into existence, 

 in which each fibre sooner or later participates. During the very brief 

 period occupied by the spread of the contraction in the cardiac tissue 

 (about oV second), there may be a change of form due to the action of the 



excited fibres on those which are 

 as yet relaxed, but it may be con- 

 sidered certain that all eventually 

 occupy the same space as they did 

 in diastole. In the case just 

 described the effort is isometric. 

 It corresponds to the isometric 

 response of skeletal muscle. If 

 the resistance is due to a constant 

 lateral pressure exercised by the 

 blood against the wall of the ven- 

 tricle, which the ventricle is able 

 to overcome, the organ contracts 

 under iso tonic conditions. 



The isotonic contraction of 

 the ventricle is best observed by 

 a plethysmograph, 1 the changes of 

 volume being measured by a piston- 

 recorder, or by Marey's tambour, 2 

 or they may be recorded photo- 

 graphically. The curve so obtained 

 corresponds, as has been shown by 

 Otto Frank 3 in a series of import- 

 ant researches, to the ordinary isotonic myogram, and is modified by 

 mechanical conditions, in ways already studied in relation to skeletal 

 muscle. 



The form of the isometric curve of the ventricle is shown in Fig. 248, a. 

 It is obtained by arranging the heart as shown in the diagram (Fig. 247). 

 As in ordinary muscle, the height of the curve, i.e. the maximum 

 pressure attained during the response, varies with the length of the 

 muscular fibres when relaxed, i.e. with the previous fulness of the 

 ventricle. In a good-sized ventricle, if the initial content is very small, 

 this maximum is correspondingly inconsiderable, but when the initial 

 capacity is increased, say to a third of a cubic centimetre, the maximum 

 may attain to over 60 mm. Beyond some such point as this, Frank 



1 Schafer, Journ. PhysioL, Cambridge and London, vol. v. p. 130. 2 Ibid. p. 127. 



3 Frank, "ZurDynamik desHerzmuskels," Ztschr.f. Biol., 1895, Bd. xxxii. S. 370, and 

 "Die Wirkung von Digitalis auf das Herz," SUzungsb. d. Gesellsch. f. Morphol. u. PhysioL 

 in Miinchen, Heft 2, S. 30. 



FIG. 247. Diagram of Frank's apparatus for 

 recording isometric curve of frog's ventricle 

 with different initial impletions. Tap I. is 

 closed and the heart allowed to empty 

 itself. Then Tap II. is closed and tracings 

 are taken with Manometer II. with no im- 

 pletion, and then with gradually increasing 

 impletion a known quantity of circulating 

 fluid being let in from the graduated tube 

 on the right between each two successive 

 observations. Frank. 



