MANUAL OF CATTLE-FEEBIISra. 77 



The Heart. — The movement of tlie blood tliroiigli tlie 

 body, in order that all organs may receive from it their 

 necessary nourishment, is accomplibhed by the heart. 



The heart is an irregularly eonical-tehaped organ, com- 

 posed of involuntary nmscles. It is situated in the ante- 

 rior part of the chest, and hangs free in an envelope called 

 the peTwanlium, 



It is divided by an impervious partition into a right and 

 left half, and each of these is subdivided by a cross-par- 

 tition into two chanibers, communicating with each other 

 by a valve in the dividing wall. The upper and smaller of 

 these divisions are known as the right and left auvide^y 

 and the lower and larger as the right and left m7it7'icles. 

 Into these divisions open several large blood-vessels, whose 

 mouths are closed with valves so arranged that the blood 

 can only flow mto the auricles and out of the ventricles. 



The blood returning from the extremities of the body 

 to the heart enters first the right auricle, a (Fig. 4), 

 through two large \eins, the vena cava anterior^ 1% coming 

 from the anterior, and the veiia cava jmste/nor^ ?, from the 

 posterior part of the body. The auricle then contracts, 

 and the blood, being prevented from returning into the 

 blood-vessels by the valves at their mouths, is forced 

 through the valve in the partition wall into the right ven- 

 tricle, &. This, in its turn, contracts, and the blood, pre- 

 vented as before by a valve fi-om turning back in its 

 course, is pressed out of the ventricle through the pul- 

 monary artery, ^, which divides into two branches leading 

 to the right and left lungs, d^ d. The opening of this 

 blood-vessel, like that of the others, is provided with a 

 \alve, which prevents the return of the blood. The blood, 

 after having been purified ni the lungs, returns to tlie hft 

 auricle^ ji^, thiougli the jmlmonary veins, represented by e. 



