460 THE ANATOMY OF THE HOUSE 



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 distinction only api)lies 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 open- 

 ings 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 efiected in the 

 passage of the blood. The force with which the left ventricle contracts 

 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 pax-t of the body to those 

 of the lower in little more than twenty seconds. Now, as this transmission 

 can only take place thx'ough 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 pericardium. 

 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. 69, at 

 page 457. It presents an irregular cone, with the base turned upwards, and 

 the apex directed towards the sternum. It is about ten and a quarter 

 inches from the base to the apex, seven inches in its antero posterior dia- 

 meter, and five and a quarter from side to side. In weight it varies from 

 six and a half to seven pounds ; but these dimensions can only be taken as 

 an approximation to the actual average. The right auricle and ventricle 

 are directed forwai'ds, 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 



