GENERAL CONSIDERATIONS
The thorax of the infant is shaped somewhat different from that of the adult, being more cylindrical, the antero-posterior being nearly the same as the transverse diameter. The transverse diameter begins to increase about the third year and this continues until puberty when the typical conical or dome-shaped thorax is attained.
In the infant and young child the walls of the thorax are exceedingly yielding and elastic. This is because the greater portion is made up of cartilaginous tissue before the completed ossification of the bony structures has taken place.
The thoracic muscles are imperfectly developed. This makes the thoracic walls very thin. In well nourished infants the walls are made thicker by the abundance of fat which is found deposited on them. The diaphragm is very high in the infant and this greatly decreases the capacity of the thorax as well as does the frequent distention of the stomach and intestinal tract because of the accumulation of gas. The trachea and bronchi of the infant are comparatively larger than in the adult, the air cells are much smaller, and for this reason a slight acute congestion will interfere almost as much with their function as will hepatization. This necessitates immediate action in all conditions which involve the respiratory tract, and especially those which affect the lungs or bronchi. In all such cases results are obtained very quickly under adjustments. In the child there is a greater tendency for the inflammation to spread in the lung tissue than in an adult.