In men it is larger than in women; hence their voice is rougher; which generally increases by exercise of the voice also. Its formation is very complex, and too tedious here to enter upon; we will therefore pass it over, and hereafter say as much as relates to its preservation, and giving it relief when disordered. ------------------------------------------- SECT. IV. _Of the Circulation of the Blood._ We shall now consider the circulation of the blood; the grand source of our existence. At the contemplation of which we must be lost in admiration, at the wonderful wisdom of its contrivance; and with uplifted eyes adore the cause of its motion! The Heart, as I before observed, has two chambers or _ventricles_, distinguished by the right and left; each of these chambers have also a valvular cavity call’d _auricle_, or ear. Each _ventricle_ opens itself into an _artery_ and _auricle_; and each _auricle_ opens itself into a _ventricle_ and _vein_. These openings have valves, which open and shut alternately, and by this, prevent any confusion or hindrance in the influx and efflux of the blood. The right _ventricle_ is the longest and greatest, but weakest: From this ventricle springs forth the _pulmonary artery_, which spreads itself throughout the Lungs. 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_.