(b) The posterior or vertebral region. In the well-developed thorax the spines of the dorsal vertebrm lie in a deep groove formed by the great mass of the erector spina; on either side. These masses extend outwards to about the posterior angles of the ribs, which are nearly in a with the spinous processes of the and may be easily distin guished, bounding the posterior region.
(c) The lateral or costal region is composed of the bodies of the ribs and the intercostal spaces. Much difference has prevailed amongst writers upon various points con nected with the ribs, and the spaces be tween the ribs. Probably this may have arisen from their having made their observa tions upon the dead subject, wherein the ribs have collapsed to their minimum, or the thorax of a thin subject with the ribs expanded by, perhaps, 200 cubic inches of air, or the ribs when artificially articulated in the clean skele ton, wherein they are generally arranged too wide apart and too horizontal.
In the collapsed state of the ribs, i. e. after the most ample expiration by death, we no tice 1st, That the intercostal spaces are not of a uniform width from the vertebra; to the ster num. They are most narrow behind, and be come wider as they approach the sternum (fig. 660.). The 1st, 2d, and 3rd spaces upon the whole are broader than the five next in ferior, and the 10th and I 1th spaces are the widest (fig. 682.).
2dly, Their length differs with the length of the ribs ; thus the two first and two last spaces are the shortest.
3dly, The position of the intercostal spaces is very oblique (fig. 684.), roughly speaking, about mid-way between the horizontal and the perpendicular.
4thly, They are moveable, and in the dif ferent stages of respiration they can assume different positions and different perpendicular widths.
It has been found necessary for the con venience of auscultation, to subdivide the thorax more minutely. The subdivision pro posed by Laennec has been generally adopted, although some of the terms, like those of the respiratory sounds, have been modified by writers of later date.
The anterior and lateral superior division, give the clearest sound upon percussion, and the back the least clear. Below Harrison's line, there is little or no evidence, from percussion, of the presence of lung, except in deep inspiration, and then the sound on per cussion is mixed with other sounds from the abdominal organs. Only a small part of the mammary region covers lung, scarcely any being subjacent to the anterior parts of the 6th, 7th, and 8th ribs. (See fig. 4, wherein a transverse shading indicates the diaphragm.) Internal conformation of the thorax. — To comprehend clearly the internal shape of the thorax, it is necessary to take a cast of this cavity. This may be done by making an open
ing in the sternal region, just large enough to admit the hand to remove the heart and lungs, filling up the whole of the cavity with plaster of Paris and returning the sternum, then opening the abdomen, cutting away the diaphragm, and so removing the cast, which gives us a clear knowledge of the internal conformation, and every kind of measurement correctly. (Fig.667. is a cast from the cavity of figs. 680-685, — male subject.) We have stated that the perfect thorax viewed externally, and the same when the superficial muscles and upper extremities are removed, differ widely in their relative form. The tho racic cavity likewise differs remarkably from either of these views. ' The general view is that of a cone, but broader from side to side than from before backwards: therefore the cone is somewhat flattened.
(a) Anterior region.— This very closely re sembles the anterior region of the external surface, being convex in its form, precisely corresponding with the concave sternum. The upper part, immediately behind the superior end of the sternum, is rather concave (fig. 667). At this part the cavity divides into two small cones, for the reception of the right and left apices of the lungs, which ascend upwards to a variable height, an inch or an inch and a half. Both apices of the lungs are directed from below, upwards and forwards; it is incorrect to consider them as directed backwards ( fig. 667. a, which is the left apex seen above the 1st rib.) In some cases, particularly where the lung presents a puckered appearance, the axis of the apex is inclined nearly to the horizontal, and at all times it is about per pendicular to the sternum. These apices, throughout advancing life, are tending to in cline forwards; it is this portion above the 1st rib (fiR.667.), which is so vulnerable in phthisis pulmonalis. There is great difference in the precise character of these apices in different individuals; in fact we have seen no two the same; some are remarkably truncated, and broad from before backwards ; others are thin in this direction, and pointed; some obtuse and low, others acute and high. They have the same characters in both sexes. There is little difference in the height of the two apices. The elevation of the liver on the right side does not necessarily cause the right apex to be the higher. The right lung is more shallow than the left ; but this is not because it is " pushed up," but because, in order to ac commodate the liver, there is less lung substance on the right side. If the mean of a series of observations represents the right side of the thoracic cavity as equal to 151, the left may be given as 182. The same may be said of both sexes.