The subject of children's cradles, baby-baskets, baby-boards, and the methods of manipulating and carrying the infant in connection therewith, have been treated of in great detail by Ploss (325), Pokrovski, and Mason (306), the second of whom has written especially of the cradles in use among the various peoples of European and Asiatic Eussia, with a general view of those employed by other races, the last with particular reference to the American aborigines. The work is illustrated, as is also that of Ploss, with many engravings. Professor Mason thus briefly sums up the various purposes which the different species of cradle subserve (306. 161-162):-- "(1) It is a mere nest for the helpless infant. "(2) It is a bed so constructed and manipulated as to enable the child to sleep either in a vertical or a horizontal position. "(3) It is a vehicle in which the child is to be transported, chiefly on the mother's back by means of a strap over the forehead, but frequently dangling like a bundle at the saddle-bow. This function, of course, always modifies the structure of the cradle, and, indeed, may have determined its very existence among nomadic tribes. "(4) It is indeed a cradle, to be hung upon the limbs to rock, answering literally to the nursery-rhyme:-- 'Rock-a-bye baby upon the tree-top, When the wind blows the cradle will rock, When the bough bends, the cradle will fall, Down will come baby, and cradle, and all.' "(5) It is also a playhouse and baby-jumper. On many--nearly all--specimens may be seen dangling objects to evoke the senses, foot-rests by means of which the little one may exercise its legs, besides other conveniences anticipatory of the child's needs. "(6) The last set of functions to which the frame is devoted are those relating to what we may call the graduation of infancy, when the papoose crawls out of its chrysalis little by little, and then abandons it altogether. The child is next seen standing partly on the mother's cincture and partly hanging to her neck, or resting like a pig in a poke within the folds of her blanket." Professor Mason sees in the cradle-board or frame "the child of geography and of meteorology," and in its use "a beautiful illustration of Bastian's theory of 'great areas.'" In the frozen North, for example, "the Eskimo mother carries her infant in the hood of her parka whenever it is necessary to take it abroad. If she used a board or a frame, the child would perish with the cold."