Inch, s. The twelfth part of a foot.
Incision, s. A cut, a wound made with a sharp instrument.
Incisor, s. Cutter, tooth in the forepart of the mouth.
Incorporate, v. To mingle different ingredients so as they shall make one mass; to unite, to associate, to embody.
Incubation, s. The act of sitting upon eggs to hatch them.
Incubation.—It is probable birds are endowed with an instinctive power of regulating the necessary heat for this purpose; of course, should the heat of the air, together with the natural warmth of the body, on the close contact of the bird to the eggs, be too great, her feelings would dictate the necessity of leaving them for a time to cool. At the early period of incubation birds quit their eggs more frequently than at the time the fetus is more perfect. Yet, in the advanced state, the embryo young is not in more danger of being destroyed, if so much; for we have frequently found a living fetus in an egg that has been taken from the nest two days. If, however, the young is within a few hours of being excluded, and the egg is suffered to be some time cold, it either dies, or becomes so weak as not to be able to extricate itself from the shell. Various degrees of heat will enlarge the embryo young, but regular heat seems necessary to its production; and yet artificial heat, regulated by the brooding of a bird, will not produce young with such certainty. In Egypt, a vast quantity of eggs are hatched by artificial heat in stoves. It is probable, however, one third or one fourth miscarry. The necessary heat for this purpose is about ninety-six degrees of Fahrenheit’s thermometer, or thirty-two of Reaumur’s scale. Birds frequently turn and change the situation of their eggs in the nest; besides this, it is possible that the moisture of the bird’s body may assist the natural growth of the fœtus and the production of the young.
The male birds of some species supply the place of the female on the nest; but then it is of short duration, and rarely, if ever, when the eggs are near hatching; at that time the female is frequently fed by the male. This is not common to all species, but very conspicuous in the rook, the pigeon, and many others. Many species of birds possess a reservoir for food, called a craw, or crop; this seems to answer the same purpose as the first stomach in ruminating animals. Here it is the food is softened and prepared for the stomach; from this reservoir it is by some ejected for the purpose of feeding their young; conspicuous in the pigeon.
The rook has a small pouch under the tongue, in which it carries food to its young. It is probable the use of the craw may be extended further than is generally imagined; for besides the common preparation of the food to assist its digestion in the stomach, there are some species that actually secrete a lacteal substance in the breeding season, which, mixing with the half-digested food, is ejected to feed and nourish the young. The mammæ, from which this milky liquor is produced, are situated on each side the upper part of the breast, immediately under the craw. In the female turtle dove we have met with these glands tumid with milky secretion, and we believe it common to both sexes of the dove genus. The cormorant or pelican genus possess no craw; but, to supply its place, they have a loose skin at the base of the under mandibles, capable of great distension, in which they carry fish to their young. The bustard is said to possess a bag of an astonishing size, for the purpose of retaining water; but the most unaccountable and extraordinary formation in the trachea of many of the males of the duck genus, called a labyrinth, is beyond our reach to discover the use of, as well as the singular flexure in the windpipe of the hooping swan and crane.
Attention during Incubation.—There is this distinction in the hen: in some, the desire of sitting or incubation is predominant, which they will repeat to the fifth or sixth time in the year, to their emaciation or almost destruction: in others the desire is so slight, that they will probably sit but twice, or even once in the season, and then not steadily. It is for the skilful breeder to take advantage of this variation of quality, the one kind furnishing plenty of eggs for the other to sit upon.