4OO THE SKIN AND ITS APPENDAGES. 



and the hair. The secretion contains fatty globules of varying size, 

 which occur either free or attached to cellular detritus. 



3. The Mammary Glands. The mammary glands are also 

 included among the cutaneous glandular structures. They are 

 developed early, but not until the fifth month is it possible to dis- 

 tinguish a solid central portion, with radially arranged tubules 

 terminating in dilatations. The structures are all derived from the 

 basal layers of the epidermis. From birth to the age of puberty 



Fig. 324. Model of a small portion of a secreting mammary gland ; X 2O - 

 (Maziarski, Anatomische Uefte, vol. xviu.j 



the organs are in a state of constant growth, and are early sur- 

 rounded by a connective-tissue sheath. The alveoli, which have 

 been developed in the mean time, are still solid and relatively small. 

 Up to the twelfth year the glands remain identical in structure 

 in boys and girls. In the female the mammary glands continue to 

 develop from the age of puberty ; in the male, on the other hand, 

 they undergo a retrograde metamorphosis, ending, finally, in the 

 atrophy of all except the excretory ducts. The mammary glands 

 do not attain their full stage of development in women until the 

 last months of pregnancy, and are functionally active at parturition. 

 The human mammary gland when fully developed has the fol- 

 lowing structure : It consists of about twenty lobes, separated from 

 each other by connective-tissue septa. These lobes are again 

 divided into a larger number of lobules, and these in turn are com- 

 posed of numerous irregularly round or oval or even tubular al- 

 veoli. The alveoli are provided with small excretory passages, 

 which unite to form the smaller ducts, these in turn uniting to form 

 the larger ducts. Shortly before terminating at the surface of the 

 mammilla, each mammary duct widens into a vesicle, the sinus 

 lactiferus. The number of excretory ducts corresponds to that 

 of the larger lobes. The ducts are lined by simple cubical epithe- 



