THE HUMAN HEART. 
357 
i 
I 
guarded the aortic opening had a large hole through the middle 
of it. 
The muscular fibres, of which the substance of the heart is 
composed, and by the contractions of which its force is exerted, 
are very much interlaced, but the greater number of them 
are inserted, by both ends, into strong rings, of fibrous 
and cartilaginous structure, which constitute the margins 
of the openings from the auricles to the ventricles, and also 
from the ventricles to the two great arteries. The fibres which 
pass round the cavity of an auricle, and so form its body, are 
inserted by both ends into the cartilaginous ring which is 
between this auricle and its corresponding ventricle, and those 
which similarly form the corresponding ventricle are inserted 
into the same ring, just as in a balloon the cords which surround 
the balloon and those which come from the car are inserted 
into one and the same ring placed between the two. Some of 
the fibres of the ventricles are also inserted into the rings at 
the openings into the two great arteries, so are the arteries 
themselves, as well as all the valves above mentioned. In some 
of the larger animals, as the ox and the elephant, there is even 
bony structure connected with some of these rings. There are 
other muscular fibres which are circular, their ends being, if we 
may so speak, inserted into themselves, like the horizontal cords 
on a balloon ; but to enter further on the arrangement of these 
fibres would be too technical for our present article. 
Now, it is evident that, owing to all tho motion involved in 
these contractions and dilatations of the heart,— motion to such 
an extent as even to make its pulsations visible externallybetween 
the ribs, — there would be a great amount of wear and tear, and 
friction against other organs, and so impediment to the heart’s 
motion itself, and injury to it and other organs, if there were not 
some contrivance to obviate this result. Accordingly we have 
the heart completely enclosed within a beautiful bag, inside which 
it can work freely, without any inconvenience or danger to itself 
or the neighbouring organs. The structure of this bag, or closed 
sac, is admirably adapted for allowing freedom of motion. It 
consists of two membranes, which adhere closely to each other 
for a great part of their extent. The outer membrane is the- 
strongest, and is continuous everywhere all round the heart, 
except where it is pierced by the lower great vein from the body ; 
it forms a kind of sheath for all the other large vessels, till, at a 
short distance from the heart, it becomes lost on their coats. 
The inner membrane is very smooth and glistening, and after 
lining the greater part of the outer one, it leaves it near the 
great vessels, and attaches itself to the coats of these, and 
accompanies them for about two inches till they enter the heart, 
when it attaches itself closely to the outside of that organ, 
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