322 THE HEAKT. 



death, were believed by the ancients to contain air; they are 

 the arteries which subdivide into very minute vessels of hair- 

 like fineness the capillaries; the latter in their turn join or 

 coalesce so as to form the veins, through which the blood 

 passes back to the heart. Every animal may be compared to 

 a sponge for its porosity, and is saturated by liquid blood, 

 which runs a definite course in accordance with the disposi- 

 tion of vessels, and a propelling organ, the heart. 



THE HEART. 



Usually described as a hollow muscle or a saccular organ, 

 provided with muscular walls, the heart is in reality a modi- 

 fication of the vessels at a definite spot, to ensure a regular 

 action, whereby the blood is impelled in the circulatory ap- 

 paratus. Its position is constant, just like the position of the 

 stomach, liver, and other important viscera. It is somewhat 

 regularly cone-shaped, with the base upwards, connected by 

 blood- vessels with the spine, and the apex downwards and back- 

 wards. In man it is situated in the left side of the chest, but 

 in quadrupeds it is as nearly as possible in the centre or the 

 chest, though its impulse is felt on the left side from the rotatory 

 movement of the organ in action. Thus lodged in the thorax, 

 between the lungs, it is enclosed in a special sac pericardium 

 which is continuous above with the large vessels, and 

 attached below to the sternum or brisket. The sac is com- 

 posed of a dense fibrous membrane, lined by a delicate serous 

 tunic which is reflected over the heart. Its relation to the 

 latter organ has been compared to the manner a double 

 night-cap covers the head. The inner layer is firmly adherent 

 to the heart, the outer layer to the fibrous sac, and there is 

 an intervening space pericardia! cavity in which a little 



