ANATOMY. 
tube, which is placed between the cricoid 
cartilages, and the origin of the bronchi. 
It is a cylindrical membranous tube, of from 
5 to | ths of an inch in diameter. It runs 
along the middle of the forepart of the neck, 
having the large blood-vessels of the head 
on each side, and being connected behind 
to the oesophagus. Soon after it has entered 
the chest, it divides into the two bronchi. 
The tube of the trachei is furnished with 
hoops of cartilage, by which it is kept per- 
manently open for the passage of the air : 
these are not however complete circles, 
being deficient behind. The lining of the 
tube is highly vascular and sensible, and co- 
vered with a copious mucous secretion, 
which is rendered necessary by the con- 
stant current of air to which it is exposed. 
The bronchi are merely the two branches 
into which the trachea divides for the two 
lungs ; and of these the right is the largest 
and shortest. They ramify through the 
lungs, dividing into smaller and smaller 
branches; and the ultimate ramifications 
communicate with the air-cells. 
ORGANS OF CIRCULATION. 
The heart is the centre of the circulat- 
ing system ; being the source of the arteries, 
and the termination of the veins. The 
younger the subject, the larger is the heart 
in proportion to the body. It is often 
smaller in tall and strong men, than under 
different circumstances. 
It is connected at its posterior part, be- 
hind the sternum, by the large blood-ves- 
sels, being unattached every where else, 
and merely confined in its situation by the 
pericardium. 
The pericardium is placed in the cavity 
of the chest, behind the second, third, 
fourth, and fifth ribs of the left side. It is 
covered to the right and left by the bags of 
the pleura, which adhere by a loose cellular 
membrane. It is not actually connected by 
any part of its surface to the sternum. Be- 
low, it rests on the diaphragm, and adheres 
very firmly to the superior surface of the 
tendon of that muscle. 
The cavity of the pericardium is larger 
than the heart, so that this viscus can move 
freely in it. 
The bag of the pericardium in shape re- 
sembles the figure of the heart itself, being 
conical. Its substance is thick and com- 
pact, and it is much more dense and strong 
than the peritoneum or pleura. Where the 
great vessels are connected to the heart, 
this membrane becomes reflected over its 
surface; and hence the substance of the 
heart has a close investment from this mem- 
brane, besides being contained loosely in 
the bag-like portion. A. small portion of 
the large blood-vessels is included within 
the cavity of the pericardium ; particularly 
of the aorta and pulmonary artery ; which 
are consequently covered by the reflected 
portions. 
The internal surface of the pericardium 
is moistened by a serous secretion from the 
exhalant arteries ; which is collected after 
death into a few drops of a clehr light yel- 
low liquor. It is an unnatural increase of 
this that constitutes dropsy of the pericar- 
dium. This fluid in the living state lubri- 
cates the opposed surfaces of the heart and 
pericardium, and thereby facilitates their 
motion on each other, and prevents their 
accretion. 
The heart, which is contained almost en- 
tirely in the left side of the chest, resembles 
a half cone ; hence we dj^fguish in it a 
basis or broad part, and an apex or nar- 
rower portion ; a convex and a flat surface. 
The basis is placed towards the right and 
backwards ; the apex points obliquely to 
the left, forwards and downwards. The 
basis is opposite to ,the seventh or eighth 
vertebra of the back, and the apex points 
to the cartilage of the fifth or sixth left rib. 
The position however varies by the motion 
of the diaphragm in respiration, as it is 
drawn down in a strong inspiration, and 
again rises in expiration. Its position also 
seems to vary slightly, according to the si- 
tuation of the body in lying. 
A small portion of the left lung seems, as 
it were, removed just at the apex of the 
heart ; so that that part of the viscus is not 
covered by the lung like the rest ; but 
touches the front of the chest. 
Those cavities of the heart which are 
called the right, are placed in front ; and 
the left cavities are towards the back part ; 
so that the epithets anterior and posterior, 
would correspond more nearly with the true 
position of these parts, than those of right 
and left. 
The flat surface of the heart looks directly 
downwards, and rests on the tendon of the 
diaphragm ; this, therefore, in point of po- 
sition is inferior ; the convex surface is turn- 
ed upwards, forwards, and obliquely towards 
the left, so that it may be called the supe- 
rior surface. 
The weight of the human heart, when re- 
moved from the body, with its pericardium, 
is from 10 to 15 ounces. 
Like the heart of all warm-blooded aai- 
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