170 



PHYSIOLOGY OF FARM ANIMALS 



[CH. 



In these cells granules (containing a j)rotein material) and fat 

 globules accumulate, and afterwards form the milk constituents 

 which the cells discharge into the lumina of the alveoli. The so- 

 called 'colostrum corpuscles' which occur in the milk in the first 

 few days after parturition or shortly before are jjrobably white 

 corpuscles since they have been observed to undergo amoeboid 

 movement. The milk constituents which are formed during 

 lactation are discharged into the lumina of the alveoli without the 

 cells which excrete them beconing detached or destroyed in the 



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Fig. 83. Section of lactating gland (from Marshall), a alveolus, b duct. 



process. The milk thus j^roduced passes from the alveoli into the 

 ducts and sinuses whence it is drawn oft by the teat being sucked. 

 In the pig the tissue lining the ducts of the mammary glands 

 may contain black or coloured pigment. This is clearly related 

 to the pigment of the hair, since it is only present in the 

 coloured breeds of pigs. It occurs in both normal and spayed 

 sows, and occasionally' also in the hog in which rudimentary 





