98 Anatomy and Physiology of the Mammary Gland. 



pregnancy the mammary glands are gradually increasing in size, 

 arising from a greater flow of blood to them, and a consequent 

 augmentation in the calibre of their bloodvessels and lacti- 

 ferous ducts. As the period of parturition approaches, they 

 are so much enlarged as to have acquired nearly their fullest 

 dimensions. The skin is also tense, red, and hot. At the time 

 of calving a fluid can be drawn from the teats which has many 

 of tlie ])roperties of milk, but differs from it in colour, being of a 

 brownish-yellow hue, and possessing also a thicker consistence. 

 Within a few days this colour and increased viscocity have 

 disappeared, and true milk is now present. The glands are per- 

 haps as large or even larger than they were at the time of par 

 turition ; but the heat, redness, and tense condition of the skin 

 have subsided : in short, the full secretive power of the mammae 

 is established, and the increased quantity of blood on which these 

 symptoms depended is now consumed in the formation of milk. 



From this period the animal daily yields her wonted supply until 

 the time of her having a second calf is drawing near, when the quan- 

 tity gradually diminishes, and on the mere suspension of the act of 

 milking passes entirely away. Her second delivery is accompanied 

 with the same train of symptoms, which are also associated with 

 the production of a peculiar fluid by the mammae, preceding 

 by two or three days the secretion of pure milk. Again, as her 

 third delivery approaches, the milk lessens in quantity, ceasing 

 ultimately to be produced as before. These phenomena are 

 repeated with each calving, and they show how much the 

 quantity of milk which is yielded by an animal is governed by 

 the states of pregnancy and parturition. 



The fluid wliich we have spoken of as appearing in the 

 udder at the time of each successive delivery is called colos- 

 trum. Microscopically examined it is found that in addition 

 to the ordinary milk globules, granular corpuscles of a pale 

 3'ellow colour are present in great abundance. They are much 

 larger than the true milk globules, measuring from ^(^(jth to 

 ^-^th of an inch. (See Fig. 8, a, a.) The existence of these 

 granular cells imparts a purgative action to the fluid, which is 

 generally viewed as being of great use in assisting in the ex- 

 pulsion of the meconium from the intestines of the young animal. 

 This peculiar substance accumulates in considerable quantity 

 in the intestinal canal dui'ing the latter period of utero-yestatiou ; 

 and it would appear that alter birth the stimulating or cathartic 

 action of the colostrum is required for its removal. 



The colostric condition, however, of the milk will often 

 remain for some time after parturition, and if so it is likely 

 seriously to affect the health of the offspring. Many fatal 

 cases of diarrhoea in very young calves, lambs, and pigs are 



