FOR many years we have jointly examined patients suffering from conditions generally believed to be associated with mouth-breathing and nasal obstruction. The observations recorded in this paper are based almost entirely upon our clinical experience. The constant use of the nose for respiration.-In mammals, birds and reptiles, the nose is the portal for entry of air in the respiratory system, whilst the mouth, the portal to the digestive system, may be used to supplement the respiratory function of the nose, under certain conditions. We wish to emphasize the fact that in man, nose-breathing is normal in all races, although the use of the mouth to supplement normal respiration occurs during severe muscular exertion and also in special acts such as yawning, coughing, sneezing and crying. Under conditions which demand rapid filling of the lungs with air, inspiration through the mouth is more effective than through the nose, as can be readily demonstrated. When respiration takes place with the mouth open, the position of the soft palate and tongue varies, according to whether the nose or mouth is mainly used. If the base of the tongue is raised and the soft palate depressed, the air enters through the nose, but if the tongue is depressed and the soft palate raised, the air enters through the mouth. Our observations have led us to believe that, except in a few rare pathological conditions, the nose is always used for respiration although at times supplemented by the mouth. The powerful urge to breathe through the nose is present in all individuals. It is most marked at birth and would appear to diminish in old age. We have made use of a strip of thin paper about one-eighth of an inch wide and two or three inches long, to determine the character of the breathing, one end of the strip being held by the hand and the other placed close to each nostril and before the mouth, alternately and repeatedly. In children we have found it useful to distract the attention by pretending to test the eyesight and asking the child to follow the same or a second strip of paper with his eyes while the test is being carried out. We have examined fifty-three infants, aged from 1 to 14 days, while asleep, in the maternity wards of the Middlesex Hospital. In every case breathing took place exclusively through the nose. One of these children was sucking two fingers and another the thumb. Several had their lips apart. In some, but not all, of these last, the oval or triangular space between the lips was plugged by the tip of the tongue. All these babies had well-developed ala nasi muscles. The mandible and lower part of the JUNE-ODONT. 1