ANATOMICAL AND PHYSIOLOGICAL PECULIARITIES OF THE INFANT nasal passages and the nasopharyngeal space are very narrow in the newborn and in nurslings. The nasal cavity is shallow corresponding to the low facial part of the skull, so that even in slight tumefactions of the nasal mucous membrane, the entrance of air is interfered with or even prevented. The contiguous nasal cavities are scarcely marked in the newborn (that of the superior maxillary bone being still absent), and develop only with age according to the standard of development of the facial part of the skull. The shape of the infan tile thorax deviates materially from that of the adult. This is shown most distinctively in the newborn and younger nurslings. While the thorax of the adult resembles an ovoid flattened postero-anteriorly with its wider portion directed downwards, that of the newborn is obtusely conical, expanding uniformly towards its base. The antero-posterior diameter is as long or almost as long as the transverse diameter, so that the chest appears well arched in front, while the lateral portions are flattened. The upper thoracic aperture is not well inclined towards the front as in older children and adults, but extends in an almost horizon tal direction. Correspondingly, the breast bone is higher by about a body and a half of a vertebra in its position to the vertebral column. The course of the ribs, in an almost horizontal direction, and the obtuse epigastric angle of the lower thoracic aperture, are also characteristic. Owing to these characteristics, the thorax acquires its short appearance and assumes to a certain extent a posture of permanent inspiration. In contrast to the adult, the transverse processes, as well as the ribs as far as the angulus, actually leave the vertelwe in a straight transverse direc tion. The ribs are soft and pliable, the thoracic muscles weak. These conditions gradually change with the growth, and lead to the permanent shape of the thorax which is already manifest at the fourth or fifth year of age. The ribs grow out from the transverse processes to the rear and then sharply to the side, so that the thorax becomes more wide than deep, making room posteriorly and laterally- for the growth of the lungs.
At the same time the upper thoracic aperture with the sternum and all the ribs descend, bringing about the broader and larger thorax with the anteriorly inclined ribs of later life.
At birth, the circumference of the chest of the newborn is about 32-33 cm. measured at the level of the nipples. The increase in the first year is about 12 cnt., in the second 3-4 cm., from the third to the sev enth year 1-2 cn). for each year, from the eighth to the twelfth year CID. for each year, towards adolescence (thirteenth to the fifteenth year) 3-5 cm. for each year. Accorclingly, the chest circumference at the age of five years would be 52-53 cm., at ten years 61-62 cm. In the well developed newborn the difTerence between the chest circumference 9,nd half of the body length is S-10 cm. in favor of the chest circum ference. The smaller this difference the lower the viability, which be comes questionable as soon as this difference falls below 7 cnt. The chest circumference, in vigorous infants, should exceed the circumfer ence of the head at the latest with the third year, and with the fifteenth year it amounts to one half of the body length.
From the fourth to the fifth year the right side of the chest exceeds that of the left, the difference in older males amounting to 11 cm. The lungs of the newborn begin to expand with the first inspiration; fre quently, however, several days are required for their complete expan sion. They are at first very small (see Fig. 77); and being confined in the short and narrow thorax, their space is still more encroached upon by the large heart and thymus. With the expansion of the thorax, the lungs develop rapidly during the first months, but still remain relatively small during childhood. The proportionate size of the lungs compared with the heart, is in the newborn 3.5-4 : 1, at puberty 7.3 : 1.