=Croup.=--I endeavoured to explain, a few pages back, the cause of that peculiar sound which is heard in spasmodic croup. The contraction of the opening of the windpipe changes the sound which passes through it, just as the opening or closing the keys of a wind instrument modifies the sound which it gives forth. But the windpipe is not simply a wind instrument, it is a stringed instrument too, and the strings or vocal cords, as they are termed, give forth, as they vibrate, tones now deeper, now more shrill. The action of this delicate apparatus is readily disturbed, if the nerve-supply to it is disordered by irritation in some distant organ, and then the breathing is accompanied by the peculiar sound of spasmodic croup, or in older children this may show itself in a different way, as in the loud, barking cough heard in some cases of constipation, or of disordered digestion; or another illustration of it is furnished by the loud, long breath--the 'hoop,' which gives its name to hooping-cough. But there is one sound that sometimes attends the breathing of children, which more than any other causes, and justly causes, the greatest anxiety to a mother; and that is the sound which is characteristic of croup. The word croup, which comes from the Lowland Scotch, signifies merely hoarseness in breathing or coughing, and is therefore, strictly speaking, the name of a sign of disease, rather than that of the disease itself. The peculiar sound is heard in two different conditions--the one in which a child having caught cold, instead of the air-tubes alone being affected, the windpipe, and especially its upper part, becomes congested, and the lining membrane swollen. Partly owing to this, partly owing to its nerve-supply being disturbed, the child breathes noisily and hoarsely, and the cough has a peculiar metallic clangor. In the other case there is not merely the congestion of the windpipe, the disturbed nerve-supply, and the swollen state of the membrane; but in connection with the influence of the special poison of diphtheria, a deposit takes place at the back of the throat, whence it extends to the windpipe, and in many instances even far beyond it, blocking up its canal, and mechanically excluding the entrance of air. To determine at once to which class a case of croup belongs is so far from easy, that I should advise that on the first sound of voice, or cough, or breathing resembling that of croup, medical advice should at once be sought. I dwell on the difference between the two: the first which has been called false croup, or better catarrhal croup, and the second called true croup, or diphtheritic croup, in order to save much needless apprehension to parents, in whose mind the croupy sound is invariably associated with nothing short of that most dangerous disease--diphtheria.