half therefore grow alike and thus the thorax is composed of two symmetrical halves.

We know that impairment of a portion of lung tissue is always followed by diminished respiratory expansion of the chest over the affected area. If the equal expansion over corresponding areas be made unequal by disease within the lung or of the pleura it is followed always, as we know, by thousands of clinical ob servations, by flattening of the chest wall over the impaired lung tissue. It is not the disease per se that flattens the chest wall; it is the diminution of the normal respiratory expansion force mainly that causes it. Also, we know by thousands of clinical observations that if through a period of time a portion of lung expands more than a similar portion of the other lung, the chest wall over the former will enlarge more than the corresponding area of chest wall over the latter compensatory expansion.

Fig. 1.

It is not disease alone that causes corresponding portions of the lung to expand unequally. Let a normal individual with naked chest stand in front of you. Lay the hands gently on corresponding areas of the chest, and direct the individual to breathe deeply. One feels the equal expansion of the chest beneath the right and left hands. Press very gently upon the chest with the left hand; the chest wall beneath the right hand is felt to expand much more forcibly than when there was no pressure with the left, and the expansion is greater than normal. Try this simple experiment with the hands on

Td

a

Fig. 2.

any areas. If they be corresponding ones the observation is still correct. If a normal person stands, bends the head and the body to the left so as to assume a right convex scoliosis (viewed from the back) the result upon the expansion of the chest is as follows: the left anterior half (by "anterior half" is meant the area from the mid line in front to the mid axillary line; by "posterior half," from the spine to the mid axillary line) of the chest expands much more than when standing erect, which we shall call the normal, the right anterior half expands much less than normal, the left posterior half expands less than normal. It makes no difference whether the person, preserving the assumed right dorsal scoliotic position bends in forward flexion at the same time, or in hyperextension. There is some difference in the degree of the expansions, but this does not affect the principle involved. One of the reasons for the decreased expansion of the right anterior half in this simple experiment is that by the posture the right intercostal fascia and muscles and abdominal fascia and muscles are put upon the stretch; this makes the right anterior half more tense than normal, which is added to if the head be far to the left by the sternocleidomastoid and scaleni pulling upwards upon the clavicle and first and second ribs. The chest wall over the front of the right lung becomes somewhat tense, and this condition offers opposition to the expansion over the area in question. So easily is the symmetrical expansion disturbed that this slight resistance is sufficient to cause the other side to expand more than the normal degree. The right posterior half expands more than normal because of the lessened expansion in the right

front half; the left posterior wall expands less because of the increased expansion of the left anterior half. The import of this is better understood by expressing it graphically.

In Figure 2 the dark lines represent an element of a normal person. In this element when the person is erect the respiratory expansion is symmetrical. Let the person assume a right convex scoliosis with slight forward flexion. There is immediately a rotation, great or less as the case may be, of the vertebra in the direction of the arrow a, and a tendency for the whole bone ring, the element, to so rotate. Immediately, too, as determined before, by feeling the expansion, the element is

subjected to an expansive force in the direction of the arrows b and c greater than ever before, and the expansion is not so great in directions of the arrows d and e; thus each time the person breathes the shape of the element is changed slightly to the figure illustrated by the dotted lines. This happens eighteen times per minute to all elements participating, in greater degree to the most flexible.

Notice in Figure 4 the rib prominence on the right side of the spine and the depression on the left side of the spine. Notice in Figure 5 the prominence of the left anterior half and the flatness of the right anterior half