726 A MANUAL OF VETERINARY PHYSIOLOGY 



gradually lose their function, and the secretion ceases. A cow 

 can withhold her milk by contraction of the muscular fibres of 

 the teat duct. If her calf has been brought up with her, she 

 may refuse to yield any milk unless the calf is present (see also 

 p. 565) . No animal, however, has the power of voluntarily dis- 

 charging the gland when distended, and after this has reached 

 a certain point the tension produces considerable pain. 



There are some unexplained phenomena connected with milk 

 production. A dog, for instance, may, in consequence of copula- 

 tion having occurred at the last oestrum, make her bed and pro- 

 duce milk, though pregnancy has never occurred. Such cases, 

 though rare are beyond doubt. A mule may suckle a foal. This 

 is even more difficult of explanation.* 



Two processes contribute to the formation of milk. In one 

 the cells lining the alveoli of the gland are shed bodily, and form 

 the fat of the milk, while in the other the water, proteins, salts, 

 etc., are formed from the lymph in the gland by the ordinary 

 process of secretion. The first process must be examined at 

 somewhat greater length. If the mammary gland of an animal 

 which has never been pregnant be examined, the alveoli it 

 contains are much smaller and less numerous than those of a 

 secreting gland. The alveoli of the first-mentioned gland are 

 found to be packed with small rounded cells of very slow growth ; 

 when the animal becomes pregnant the gland enlarges, the 

 alveoli increase in number, but remain packed with cells until 

 parturition approaches or occurs. The solid masses of cells 

 are now cast off, and leave behind them alveoli lined with a 

 single layer of secretory epithelium, the function of which is to 

 furnish the milk. The shedding of the mass of cells which 

 originally occupied the alveoli supplies the colostrum or first 

 milk. 4 



The appearance presented by the single layer of cells lining 

 the alvelous of the secretory gland depends upon whether the 

 gland is loaded or discharged. If the gland is loaded — viz., 

 active secretion occurring — the cells are found to be large and 

 columnar in shape, possessing two or more nuclei, one being at 

 the base of the cell, and the other, giving indications of degenera- 

 tion, placed near the apex (Fig. 257). In the apex or free 

 portions of the cell fat globules can be seen, which may even 

 have partly extruded themselves from the cell, and besides these 

 there are other particles. Further, the cell gives the appear- 



* The only case within the writer's knowledge occurred during the war 

 in South Africa. The mule and foal were found on the veldt ; the general 

 assumption was that the mule was the dam. This is out of the question ; 

 the foal on the death of its mother no doubt attached itself to the mule, 

 but the mechanism by which the mule produced milk in consequence of 

 the foal sucking the mammary glands is unknown. 



