184 



Types and Market Classes of Live Stock 



the arteries through which the blood came, and being similarly named. 

 The other route is already familiar, leading out in front of the udder 

 through the large subcutaneous abdominal veins (milk- veins) which 

 pass through the chest wall and become a part of the internal thoracic 

 vein, reaching the heart by way of the anterior vena cava. The direc- 

 tion of the blood in the encircling veins at the top of the udder is de- 

 termined by the valves in the vessels. The larger volume of blood 

 passes through the milk-veins, thus bringing them into great promi- 

 nence. During pregnancy, the pressure of the uterus tends to interfere 

 with the circulation through the posterior vessels, and this tends to 

 further increase the size of the milk-veins. Inasmuch as the blood 

 may return to the heart posteriorly, it is possible that a high producer 



Fig. 66. — Circulation to and from the udder. The broken lines represent the 

 arteries which carry blood containing the nutritive material to the udder where it 

 is manufactured into milk. The heavy black lines represent the veins which carry 

 the blood back to the heart. Note that there is but one route from the heart to the 

 udder, whereas there are two routes from the udder to the heart. M. V., milk-vein; 

 M. W., milk-well. 



may have small milk-veins, but this is not likely to occur. Cows with 

 large milk-veins are large producers, and cows with small milk-veins 

 may be large producers. 



Nature and composition of milk. — Cow's milk is an opaque, yellow- 

 ish-white fluid devoid of odor except for a short time after its extraction. 

 When fresh, it is slightly sweet and has a slightly alkaline reaction. 

 Under the microscope, milk is found to be a fine emulsion of fat, a 

 quart of milk being estimated to contain 2,000,000,000,000 fat globules, 

 and the cow that gives two gallons of milk per day must therefore 

 secrete at the rate of over 175,000,000 fat globules per second. The 



