Chap, xni.] MAMMARY GLANDS. 523 



single duct (as in Mus ; D) traverses the projection. 

 The false teat is found, for example, in the cow (T and 

 F), where the sides of the mammary pouch are drawn 

 out to form a canal, at the pit of which the mammary 

 ducts open. 



Though the mammary glands are merely integu- 

 mentary glands set apart for the secretion of a special 

 fluid, milk, they are remarkable for being unlike any 

 gland found in any other vertebrate. The characters 

 of the secretion vary within limits ; the milk of the 

 mare, for example, being poor in fats, but so rich in 

 sugar as to easily ferment, and form a spirituous 

 liquor (koumiss) ; in other cases, e.g. the goat, the 

 milk has the odour of the animal that secretes it. 

 As a rule the glands are active in females only, but 

 medical observers have put on record a few cases of 

 "male lactation." 



In the lower Mammals the number of teats is 

 much greater than in the higher forms, while their 

 presence on the thorax in some (man, Sirenia), and 011 

 the groin (cow) only in other mammals, speaks to 

 their having primitively extended along a large part 

 of the ventral surface. The Centetidse have twelve 

 pairs of teats (Dobson), the rabbit and the hedgehog 

 five. The teats are often found to correspond in 

 number to the maximum number of young pro- 

 duced at a birth ; in no known case do they exceed 

 fourteen. 



In the Marsupialia the teats are often arranged in 

 a circle round a central larger teat, and the whole 

 mammary area is enclosed by a fold of the skin, the 

 marsupial pouch, into which the young, which 

 are born in an altogether helpless condition, and at a 

 period so early that they are unable to actively suck 

 the mother, are conveyed. These helpless babes are 

 fed by the mother, who forces the milk out of her 

 mammary glands by the contraction of the cremaster 



