358 
POPULAR SCIENCE REVIEW. 
being firmly adherent to it in every part, and completely con- 
tinuous with itself; so that between the part which lines the 
outside of the heart and the part which lines the inside of the 
outer membrane, there is an empty cavity completely closed in, in 
which the heart moves about; and, to facilitate its movements 
still more, a small amount of oily fluid is secreted in the interior 
of this cavity by the shining walls of the inner, or serous, mem- 
brane. The arrangement of these membranes, which form what 
is called the pericardium, is a little difficult to explain without 
an illustration, and we well remember what difficulty we ourselves 
at first had in understanding it ; but we may illustrate it by 
comparing the heart to a hand, with a glove fitting very closely, 
or rather adhering to it, thrust inside another glove fitting very 
loosely, the wrists of the two gloves being then sewn together, 
so as to form between the two a closed sac for the hand to move 
about in ; and if we could then imagine a third glove made to 
adhere closely over the greater part of the second, but leaving 
it at the wrist, and, a little higher up, by some strange process 
losing itself by uniting with the skin of the arm, which in this 
case would represent the great vessels, the analogy would be 
complete. 
There is yet another thin and smooth membrane which lines 
tlie inside of all the cavities of the heart ; it is called the eudo- 
cardium, and is continuous with itself and with the membrane 
lining the inside of the great vessels which enter the heart ; and 
it is of this membrane, doubled on itself at the rings of fibrous 
and cartilaginous structure, and there enclosing some fibrous 
structure, that all the valves above alluded to are composed. 
Strange to say, a wound of the muscular structure of the 
heart is not necessarily fatal, even though it enter the cavities, 
provided the valves and vessels are uninjured ; the contraction 
of the muscular fibres is in so many different planes, that it may 
even close the wound and prevent bleeding. Thus there is an 
instance well known amongst members of the medical profes- 
sion in which a soldier was shot through the heart, who still 
recovered, and lived for six years, and after his death the heart 
was opened, and the bullet found in it, in the right ventricle, 
lying against the thin muscular wall between the two ventricles. 
Nothing but the result of the post-mortem examination could 
have made such a case credible. 
The development of the heart from its very earliest stage is 
interesting and remarkable. In tracing the early development 
of the higher animals, we find successive stages of progress, 
each stage corresponding almost exactly with the permanent or 
perfect state of a class of animals below that in question. 
The heart, for instance, of all vertebrate animals is at first 
very like the circulatory organ, for a heart we can scarcely call 
