(b) Posterior region. — If we examine this region upon a cast of the thoracic cavity, we observe in the middle line a deep fossa, formed by the projection, to the very centre of the thoracic cavity, of the bodies of the dorsal vertebrae.
This fossa is wider towards the base of the thorax, as may be seen by comparing the dif ferent sections of the cast (compare the notches, 10, IL 1, and 12, with those of 3, 4, and .5 in . 668.).
directions. There is no part of the skeleton which more strikingly demonstrates that man was destined for the erect posture, than this central position, together with the increasing dimensions from above downwards, of the vertebral column. These conditions exist only in the human subject. The groove in the cast formed by the dorsal vertebrm is directed upwards and forwards, so that at the apex of the thoracic cavity it completely divides the lungs, producing two little cones laterally for the lungs, forming the right and left apex.
At the base of the thorax this perpendicular column again completely divides the lower lobes of the two lungs. The shape of the posterior part of the cast is that of a curve directed upwards, and sharply forwards, near the apex (fig. 667.). This curve near the apex is, like the form of the apices, very various.
An inflated lung assumes the same shape as this cast, giving even the marks of the ribs. The student, in order to have a correct idea of the lungs, should remove them from the body with the heart attached ; then inflate them to their utmost, when their shape, their lobes, and relative mass before and behind, are clearly seen.
If now we inspect the cavity of the thorax itself, we find that the bodies of the vertebra: by their projection as above described, divide the posterior portion of the chest into two vast lateral grooves, which lodge the posterior portions of the lungs ; these two grooves, partaking of the form of the thorax, are consequently conical in their configuration. They lodge fully one half of the entire lungs. This is worthy of remembering in reference to diagnosis, particularly when pneumonia is sus pected ; in such cases auscultation of the dorsal region demands as much attention as that of the anterior region. Though less vulnerable to phthisis pulmonalis, yet it may lodge disease in parts comparatively remote, and where in flammation may insidiously gain serious ground upon the patient, particularly in children.
(c) Lateral region. — This precisely corre sponds with the external lateral view of the thorax (compare fig. 667. with fig. 680.); it presents indentation of the ribs, resembling a diagonal, curved, grate.
(d) The base of the thoracic cavity has been described above.
Conformation as afected by age and sex.
—Age alters the conformation of the chest. In the earlier periods of existence the thorax is the smallest of the three great cavities, probably from the inactivity of the lungs. In the fcetal thorax the antero-posterior dia meter exceeds the transverse diameter, the sternum projecting forwards, and the heart and thymus gland filling up the middle of the cavity. The ribs in fcetal life are less curved, and conse quently those deep grooves, seen in the interior of the chest on each side the spine, formed by the angle of the ribs, so conspicuous in after life, are almost wanting ; the vertical depth also is much less at this period, because the lungs are unexpended and unemployed, while the abdominal viscera, particularly the The bodies of the thoracic vertebra;, form almost a complete septum ; they are certainly a central column of support for the whole trunk of the body, bearing an equal distri bution of the superincumbent weight in all large liver, hre in activity and pushing up the diaphragm. The superior opening or true apex is greater from before backwards than trans versely, which is the very contrary to the adult conformation. The inferior or true base of the thorax is extremely wide in every direction, from the encroachment of the ab dominal viscera. At birth the thorax sud denly enlarges, by the air expanding the lungs to two or three times their previous cubic dimensions. As age increases, the curvatures of the ribs increase, and, with the vertebrm, running up through the very centre of the tho rax, form the two great lateral grooves, peculiar to man, for lodging the chief bulk of the lungs. The depth of the thorax is diminished, while its breadth is increased, and this participates in that more perfect development of the system at the age of puberty. It is at this time that malformation of the chest fre quently becomes obvious, particularly in females. In the adult age the thorax still grows, but in a degree less apparent, until it assumes the form of is termed an open chest, capable of expanding in any di rection, supplying us with air under violent exercise, and resisting severe blows. As age advances, through the decline of life, the thorax has a tendency to collapse ; the bony framework threatens to unite into one rigid cage, the true apex droops forward, the shoul ders appear higher, and the round back of old age becomes apparent, so that we may make a tolerable guess at the age of an individual by the conformation of the back. The erect thorax is absolutely necessary to healthy vigour, while the drooping-forward chest is always accompanied with proportionate feeble ness.