126 NUTRITION OF FARM ANIMALS 



malian heart, which is substantially the same in all farm 

 animals. 



It is divided by an impervious partition into a right and left 

 half, and each of these is subdivided by a cross partition into 

 two chambers, 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 auricles, and the lower and 

 larger as the right and left ventricles. Into these cavities of the 

 heart open several large blood vessels, whose mouths are closed 

 with valves so arranged that the blood can only flow into the 

 auricles and out of the ventricles. 



180. Arteries. The blood vessels which conduct the blood 

 from the heart to the various organs of the body are called 

 arteries and may be described as tubes with strong, elastic 

 and contractile walls, to withstand the force with which the 

 blood is pumped into them by the heart. Their walls consist 

 of an outer layer of elastic and connective tissue, a middle layer 

 of muscular tissue and an inner layer of epithelium. The ar- 

 teries originate in the aorta (h, Fig. 18), which receives the blood 

 from the left ventricle, and as they extend farther and farther 

 from the heart subdivide and throw off branches to the various 

 organs, the more minute of which are called arterioles, finally 

 ending in the capillaries. 



181. Capillaries. The capillaries are exceedingly minute 

 blood vessels which penetrate all the tissues of the body and 

 form the connecting link between the arteries and veins. Their 

 walls are thin and delicate, and through them the nutritive mat- 

 ters of the blood pass out into the tissues while the waste prod- 

 ucts of cell activity pass from the tissues into the blood. In 

 Fig. 1 8, n represents the capillaries of the posterior part of the 

 body, o those of the stomach and intestines, t those of the kid- 

 neys, p those of the liver, and m those of the anterior part of 

 the body. The capillaries gradually unite again into larger 

 vessels, the veins, which convey the blood back to the heart 

 and lungs. 



182. Veins. The veins are tubular vessels somewhat similar 

 to the arteries but with weaker and non-elastic walls, the pres- 

 sure of the blood on them being slight, owing to the interposi- 

 tion of the capillaries between them and the arteries and to the 

 fact that their total cross section is greater than that of the 



