288 THE COMPLETE GRAZIER. BOOK ir. 



Water 78'7 



Casein 7'3 



Albumin .......... 7 '5 



Fats . 4'0 



Sugar ........... 1 '5 



Salts 1-0 



lOO'O 



The source of the water of milk must undoubtedly be sought in the 

 water of the blood. As has previously been stated, the sanguineous 

 fluid oozes through the finest blood capillaries in all parts of the body, 

 and in most parts of the system this overflow is kept under control by 

 means of the lymphatic vessels which collect the thin fluid, now known 

 as lymph, and return it to the heart, though in most cases the lymph 

 passes through the thoracic duct on its way to the central organ of 

 circulation. The thoracic duct, it will be remembered, also receives 

 the chyle collected by the lacteals, or lymphatics of the intestine, and 

 it is estimated that in the course of twenty-four hours a quantity of 

 fluid equal to that of the blood is thus poured into the duct on its way 

 to the heart, about one-half of this fluid being lymph and the remainder 

 chyle. Lymph is a pale yellowish fluid, containing from 94 to 95 per 

 cent, of water, and may be regarded as blood deprived of its red cor- 

 puscles and diluted with water, while chyle may be described as lymph 

 containing much fatty matter. The great distension which the mam- 

 mary gland undergoes, as it becomes stocked with milk, is chiefly due 

 to the accumulation of water, the presence of which is necessitated as 

 a vehicle whereby the other constituents of the milk may be con- 

 veniently taken up and carried away. The copious supply of blood, in 

 the capillaries which surround each alveolus of the mammary gland, is 

 most favourably circumstanced for giving up its aqueous material 

 through the moist delicate membranes by which it is enclosed ; any 

 watery fluid which did not go to swell the excretion of milk would find 

 its way amongst the tissues constituting the framework of the gland 

 into the lymphatics, and so back by way of the thoracic duct into the 

 blood. The kidney affords another example of an organ whose secre- 

 tion, mainly nitrogenous in this case, is mingled with much water and 

 converted into an excretion. 



It is important to remember that the mammary gland of the cow, 

 particularly in such favourite milking breeds as the Jerseys, the Ayr- 

 shires, and the Dutch, has been brought into a condition of abnormal 

 activity differing as much, perhaps, from the gland in its unimproved 

 condition, as does the large shapely root of a cultivated turnip from 

 that of its wild progenitor. The duration of lactation has been ex- 

 tended, the quantity of the lacteal fluid has increased. Moreover, the 

 quality of the milk is found, within certain limits, to vary under 

 different conditions, the percentage of water sinking at times to as low 

 as 84, and rising to as much as 90. It is a legitimate and useful object 

 of inquiry as to how far this kind of variation can be controlled, and 

 particularly as to what extent it can be modified with economical 

 advantage. 



