THE HEART AND ARSLERIES. 41} 
is thus made up of two pumps, yet they are united into one organ, and 
the two auricles and two ventricles each contract at the same moment, 
causing only a double sound to be heard, instead of a quadruple one, 
when the ear is applied to the chest. In the diagram it will be seen that 
‘ one-half of the cavities and vessels is shaded, indicating that it contains 
dark blood, while the other contains blood of a bright red colour. But 
though we commonly call the one venous, and the other arterial, the dis- 
‘ tinction only applies to the general circulation ; for that of the lungs is 
exactly the reverse, the pulmonary artery (E) containing dark blood, and 
the pulmonary veins bringing it back to the heart after it is purified, and 
has again received oxygen sufficient to develop the scarlet colour again. 
Between the auricles and ventricles, and again at the openings of the 
latter cavities into their respective arteries, valves of a form peculiar to 
each are placed, so as to allow of the free passage onwards of the blood, 
but not of its return by regurgitation. If they become diseased, the 
action of the heart is impeded, and the circulation of the blood is more or 
less seriously interfered with. So, also, if the muscular fibres, of which 
the walls of the auricles and, in much thicker layers, of the ventricles are 
composed, become weak by want of proper exercise, or from the deposit 
of fat in their interspaces, a corresponding degree of mischief is effected in 
the passage of the blood. The force with which the left ventricle con- 
tracts may be estimated from the fact, that if a pipe is inserted in the 
carotid artery of a horse, and held perpendicularly, the blood will rise in 
it to a height of ten feet ; and the rapidity of his circulation is such, that 
a saline substance will pass from the veins of the upper part of the body 
to those of the lower in little more than twenty seconds. Now, as this 
transmission can only take place through the current that returns to the 
heart, and passes thence through the lungs and back again, afterwards 
being forced into the lower vessels through the aorta, it follows that every 
particle of this fluid passes completely through the whole circulation in 
the above short period of time. 
THE HEART AND ARTERIES. 
THE HEART OF THE HORSE (composed, as has been already mentioned, 
of two auricles and ventricles, with their several valves, and placed within 
the thorax in the space called the mediastinum, between{the two sacs of 
the pleura) is covered by a fibro-serous sac of its own, called the perz- 
cardium. It is situated opposite the third, fourth, fifth, and sixth ribs, 
immediately in front of the diaphragm, and above the sternum, as shown 
in Fig. 2, at page 408. It presents an irregular cone, with the base turned 
upwards, and the apex directed towards the sternum. It is about tenanda 
quarter inches from the base to the apex, seven inches in its antero-posterior 
diameter, and five and a quarter from side to side. In weight it varies 
from six and a half to seven peunds; but these dimensions can only be 
taken as an approximation to the actual average. The right auricle and 
ventricle are directed forwards, and the left backwards. The auricles 
have much thinner walls than the ventricles, and the muscular substance 
of the left ventricle, occupying the apex of the heart, is very much 
thicker than that of the right. The organ is supplied with blood for its 
nourishment by two arteries (the coronary), which leave the aorta close to 
its origin, and their trunks lie in the space on each side between the two 
ventricles. The movements of the heart may be carried on independently 
of the brain and spinal cord, if these parts are gradually removed ; but if 
