The right _auricle_ is also the largest; in which opens itself the large vein, called _vena cava_; as the common trunk that receives the blood from all its branches spread over the whole human body.
The left _ventricle_ is shorter, but the walls are considerably stronger than that of the right _ventricle_. Here begins the _aorta_, or the great artery, by which the blood is sent to all the parts of the human body.
The left _auricle_ is also somewhat smaller than the right; and in that opens itself the _pulmonary vein_, which returns the blood from the lungs, after it has received its necessary assistance from the vesicles of air in the lungs.
This premised and understood, the circulation is performed in the following manner:
The right _ventricle_ being full of blood, contracts itself. This contraction is called the _systole_, by which it forces the blood through the _pulmonary artery_ into every part of the lungs, even into every vesicle; where it receives a portion of the air, drawn in by the wind-pipe.
At the end of these _arteries_ join the ramifications of the _pulmonary vein_, which receives the blood from the ramifications of the arteries, by _anastomosation_; which, by little quantities, these veins gather gradually into little branches, and at last become one common trunk, which empties the blood into the _left auricle_ of the heart.
From the _left auricle_ it is let, by means of a peculiar constructed _valve_, into the _left ventricle_; which, by a dilation called the _diastole_, receives it from its _auricle_.
Immediately upon this reception, the _systole_ or contraction takes place, and the blood is forced into the great universal artery, _aorta_; by which it is farther transported into every part of the human body.
This _artery_ spreads as it goes, and continually decreases in magnitude, as it becomes numerous in branches; until it terminates into the smallest ramification in the remotest part of the body.
The _systolic contraction_, and the _diastolic dilation_, which alternately take place in the heart, continue throughout the arteries from the great trunk to the remotest capillary ramifications,[5] by which means the blood is transported with more certainty and facility to the minutest part.