Types and Market Classes of Live Stock 



183 



alveoli of the gland. If we pack the bunch of grapes in a small basket 

 of sawdust, so that the sawdust fills up loosely the spaces between the 

 individual grapes and the branches of the stem, we may develop our 

 comparison further; the sawdust stands for the connective tissue in 

 which the ducts and alveoli are embedded, and the basket stands for 

 the capsule. 



The arterial circulation. — The mammary glands are abundantly 

 supplied with blood. The blood leaves the heart through the posterior 

 aorta, common iliacs, and external iliac, which carry backward to the 

 region of the hips. The external iliac there divides into two arteries, 

 one of which, the prepubic, divides into the two pudic arteries, the 

 external one of which passes down the thigh and gives off a branch, 

 known as the mammary artery, which enters the top of the udder from 



Fig. 65. — Where milk is made. Group of alveoli, a, Duct; b, capillary net- 

 work; c, alveoli; d, epithelial cells; e, fibrous tissue. 



the rear. The mammary artery has four large branches, one for each 

 quarter of the udder, and there is also a small branch for each rudi- 

 mentary gland. The large branches subdivide within the gland tissue. 

 The venous circulation is more complex than the arterial. The 

 blood is collected from the capillaries by from 14 to 17 large veins 

 which empty into the mammary vein running parallel with the mam- 

 mary artery at the top of the udder. The mammary vein is divided 

 into two parts which encircle the top of the udder and connect in front 

 and behind like a rope tied around it. From this circuit of veins the 

 blood returns to the heart by two routes. One route leads out to the 

 rear of the udder, then up to the region of the hips, and thence to the 

 vena cava and the heart, the veins along the route being parallel to 



