ABSTRACT
Mammals are distinguishable by the presence in the female of mammary glands for the secretion of milk. These are skin glands which are associated functionally with the reproductive system, but there is great diversity in their morphology from species to species. Of the domesticated farm animals, the ruminants (cow, goat, or sheep, for example) have a single structure, the udder, in which the individual glands are grouped together. Yet each gland is functionally separate and milk cannot pass from one gland to another. In other species, (the sow for example) the glands are structurally as well as functionally separate.
