174 THE STOCKFEEDER'S COMPANION 



abdomen forwards till it almost reaches the posterior 

 part of the breast bone where the veins turn inwards, 

 thus leading the blood back to the heart. These points 

 of turning are called the " milk-wells." 



Theories of Milk Secretion. (a) Up to 1840, milk 

 was considered to be merely filtered blood, the milk 

 glands acting simply as a filter. Blood certainly 

 resembles milk in several respects, but there are 

 important chemical differences at least, which make 

 this theory untenable. The resemblances and differ- 

 ences are placed in apposition. 



Milk. Blood. 



Consists of fat-globules floating Consists of blood corpuscles 



in a serum (separated milk). floating in " blood " serum. 



Fat-globules give characteristic Blood corpuscles give charac- 



yellow colour to milk. teristic red colour to blood. 



Milk is 1-032 times as heavy as Blood is 1-055 times as heavy 



water. as water. 



Milk left exposed to air gener- Blood left exposed to air clots 



ally curdles or sours. through formation of fibrin. 



Soda salts predominate in ash Potash salts predominate in 



of milk. ash of blood. 



Fat in milk is comparatively Fat in blood consists largely of 



rich in volatile fatty acids. non-volatile fatty acids. 



The quantity and quality of milk has not been 

 affected very greatly when different foods have been 

 experimentally fed, so long as the ration used was 

 sufficiently rich in nutrients. This result should 

 naturally have been brought about if milk were only 

 filtered blood, but as milk constantly exhibited the 

 last two chemical differences given above, another 

 theory had to be formulated. 



() The modern theory is that milk is manufactured 

 by the mammary glands from the blood. The way in 



