ORGAZPS OF CIRCULATION.
The heart is the centre of the circulat ing system ; being the source of the ar teries, and the termination of the veins. The younger the 'subject, the larger is the heart in proportionto 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 Ef the chest, behind the second, third, Ilritrth, and fifth ribs of the left side. It is covered to the right and Aloft by the bags of the pleura, which adhere by a loose cellular membrane It is not actual ly connected by any part of its surface to the sternum.. Below, it rests on the diaphragm, and 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 resembles the figure of the heart itself, being conical. Its substance is thick and compact, 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 re flected over its surface ; and hence the substance of the heart has a close invest ment from this membrane, besides being contained loosely in the portion. A small portion of the large blood-ves sels is included within the cavity of the pericardium ; particularly of the aorta and pulmonary artery; which are conse quently covered by the reflected por tions.
The internal surface of the vm is moistened by a serous secretion from the exhalant arteries; which is col lected after death into a few drops of a clear yellow liquor. It is an un natural increase of this that constitutes dropsy of the pericardium. This fluid in the living state lubricates 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 entirely in the left side of the chest, re sembles a half cone ; hence we clistin guish in it a basis or broad part; and an apex or narrower portion ; a convex and a flat surface. The basis is placed to
wards 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 positiori bowever varies by the motion of the dia phragm in respiration, as it is dmwn clown in a strong inspiration, and again rises in expiration. ltsposition also seems to vary slightly, according to tbe situation 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 mom nearly with the true position of these parts, than those of right and left.
The flat surface of the heart looks di rectly downwards, and rests on the ten don of the diaphragm ; this, therefore, in point of position, is inferior; the convex surface is turned upwards, forwards, and obliquely towards the left, so that it may be called the superior surface.
The weight of the human heart, when removed from the body, with its pericar dium, is froth 10 to 15 ounces.
Like the heart of all warm-blooded animals, this organ consists of two hearts, closely and intimately connected. One of these is concerned with the cir culation through the body, or the greater circulation ; the other with the circula tion through the lungs, or the minor cir culation. These might perform their of fices, if separate and even distant from each other. Each of these hearts con sists of two cavities; an auricle, or mem branous bag, placed at die mouths of the veins ; ventricle, or strcmg muscular organ, placed at the orifice of the artery, and constructed for the purpose of driv ing the blood into that vessel and its branches.