406 THE AMERICAN NATURALIST. [Vor. XXXV. 
The source of the stimulus for the contraction of the heart muscle 
was sought for in several ways. An isolated heart was found to be 
capable of beating regularly in either direction. Stimulation of the 
animal's brain had no effect on the heart-beat. Removal of the 
brain reduced the rate, but this was shown to be due to the loss of 
substance suffered by the animal and not to the removal of the 
brain. The filling of the heart with blood was shown not to be 
necessary for its contraction. As small fragments of the heart 
muscle continued to contract rhythmically, and yet on examination 
showed no evidence of nerve fibres or of ganglion cells, Schultze 
concluded that the motor stimulus for the action of the heart muscle 
must be geferated exclusively by the metabolism of that muscle 
itself. 
The alternating action of the heart depends on the capacity of its 
muscle to transmit the stimulus to contraction directly from fibre to 
fibre and on the varying rhythm of the two ends of the heart. In 
moribund individuals both ends of the heart may at times give rise 
to contraction waves simultaneously. These usually meet near the 
middle of the heart and neutralize one another. In normal indi- 
viduals the rhythm at one end is so much more rapid than that at 
the other that this rhythm asserts itself for the whole heart. When, 
however, the muscle tissue of the given end becomes somewhat 
exhausted by continued action and thereby reduces its rate of con- 
traction, the muscle substance of the opposite end, having recovered 
from the effects of its own previous action, may be able to establish 
a more rapid rate than its opponent, and thus the center of propaga 
tion is transferred to the recuperated end. Hence the quiescence of 
a given end permits that end to recuperate till its own rhythm can 
supersede that of the opposing end, and its action gradually exhausts 
it so that its opponent in turn will be able to gain the ascendency. 
Flies as Carriers of Disease. — Dr. L. O. Howard, in a recent 
paper,' has presented the possibilities of the transmission of disease 
by flies in a particularly striking manner. A large number of flies, 
representing many species, were bred from human excrement. 
Those seen visiting the same material were collected. Then collec- 
tions were made in dining-rooms and pantries, and many sheets ja 
sticky fly-paper examined to see what species commonly occur in 
* A Contribution to the Study of the Insect Fauna of Human Excrement, Prec. 
Wash. Acad. Sci., vol. ii (1900), pp. 541—604. 
