THE HEART-BEAT. 
235 
height of eight or nine hundreds of miles. Yet all this is done 
so smoothly and harmoniously that we are not generally con- 
scious even of the existence of such an organ. By applying to 
the chest wall an instrument called a cardiograph we can 
magnify these movements, and by suitable apparatus obtain a 
record of each contraction M’ritten by the heart itself. By 
listening with a “ stethoscope ” we can hear the sounds made by 
the blood being driven through its chambers or stopped by its 
membranous valves, for, like other pumps, it is so constructed 
that fluid can travel through it in one direction only. In some 
simple organisms there are no valves. The heart in these is a 
mere muscular tube connected at either end with blood vessels, 
and the fluid travels first in one direction for a certain number 
of beats, and is then reversed and forced in the other. This can 
he easily seen ; and in cold-blooded animals, such as frogs and 
turtles, the heart will continue to b(‘at for hours or even days 
after its removal from the body, provided it be kept moist. 
By these and other methods of investigation a vast amount 
of knowledge has been gained as to the mechanism of the cardiac 
movements, and much light has been thrown on that most 
interesting question : “ Why does the heart beat ? ” Like most 
problems in physiology, when pushed fur enough, this baffles 
our endeavours. Dr. Brown-Sequard says that it is the presence 
of impure blood that stimulates its muscular fibres to contraction. 
Others say that it is the mere contact of fluid. Yet the heart 
will beat when empty, so this hypothesis does not hold. 
Then there is the commonly-received theory that the nerves 
scattered throughout the heart-substance, and those connecting 
it with the brain, are the cause of its movements. Undoubtedly 
they are to some extent, but not altogether, for the developing 
heart beats before it contains either nerve or muscle. Dr. 
Carpenter says : — “ The essential cause of the rhythmical con- 
traction of the heart must still remain an unsolved question.” 
