THE CIRCULATION OF THE BLOOD. 937 
in structure a position intermediate between the striped vol- 
untary fibers and the smooth muscle-cells, Numerous investi- 
gations have shown that the heart is not susceptible of true 
Fia. 211. Fig. 212. 
Fie. 211.—Microscopic appearances of fibers from the heart. The cross-strie, divisions 
(branching), and junctures are visible (Landois). 
Fic. 212.—Muscular fiber-cells from the heart. (1 x 425.) a, line of juncture between two 
cells ; b, c, branching cells. 
tetanic contraction, certainly not the heart of the mammal; so 
that it is customary to term the cardiac contraction peristaltic. 
If this view be correct, how could there bea sound produced by 
muscular contraction alone? To this it has been replied that 
the sudden tension of the ventricular wall when tightened over 
the blood may give rise to vibrations that account for the 
sound; and recent investigations have shown that the vibrations 
that give rise to the sound emitted by a contracting skeletal 
muscle may be fewer than was once supposed. The statement 
that a sound may be heard from the excised ventricle under the 
circumstances above mentioned has not been denied; but its 
source has been traced to the action of the heart wall against the 
. Stethoscope—i. e., some believe the sound to be, in this case, 
of extrinsic origin. Most physicians would be very loath to 
abandon the view that the valves are always to be taken into 
serious account as a factor in the causation of the sound.. 
But, looking at the whole question broadly, is it not unrea- 
sonable to explain the sound resulting from such a complex act 
as the contraction of the heart and what it implies in the light 
of any single factor ? That such narrow and exclusive views 
. Should have been propagated, even by eminent physiologists, 
should admonish the student to receive with great caution ex- 
