This section is from the book "The Dogs Of Great Britain, America, And Other Countries. Their Breeding, Training, and Management in Health and Disease", by John Henry Walsh (Stonehenge). Also available from Amazon: The Dogs Of Great Britain, America And Other Countries.
The duration of the period of heat in the bitch is about three weeks, during the middle week of which she will generally take the dog; but about the eleventh or twelfth day from the first ivuiiifencement is, on the average, the best time to bring her to him. During the first three or four days of the middle week the bitch "sleeds" considerably from the vulva, and while this is going on see should not have access to the male, nor will she generally, if left to herself. But as soon as it subsides, no time should be lost, as it often happens that very shortly afterwards she will refuse him altogether, and thus a whole year may be lost. Most bitches are "in heat" twice a year, at equal periods; some every five, or even every four, months; others every seven, eight, nine, ten, eleven, or twelve months; but the far greater proportion of bitches of all breeds are "in season" twice a year pretty regularly. There is, therefore, a necessity for ascertaining the rule in each bitch, as it varies so considerably; for, when it is known, the calculation can better be made as to the probability of the heat returning at the desired time.
The period between the first and second "heats" will generally indicate the length of the succeeding ones; but this is not invariable, as the "putting by" of the animal will sometimes throw her out of her regular course.
When bitches are not intended to breed, they are carefully "put by," that is to say, they are secluded from the dog, and during that time they are in great measure deprived of their usual exercise. From this circumstance they are very apt to get out of health, and some injury is thereby done to their offspring as well as themselves. At this time, from their general feverishness, as well as from their deprivation of exercise, they ought to be kept rather lower than usual, and very little meat should be given. Slops and vegetables, mixed with biscuit or oatmeal, form the most suitable diet; but, if the bitch has been accustomed to a great deal of flesh, it will not do to deprive her of it altogether. Bearing in mind then this caution, it is only necessary to remember that she must be lowered in condition, but not so starved as to suffer by the sudden change. After the end of the period, a little cooling medicine will often be required, consisting of a dose of oil or salts.