OF THE THORAX IN MAN. Anatomy of the framework of the Thorax.—A portion of the spine, the ribs, the sternum, together with numerous muscles, form the wall of the human thorax. The framework of bones is so ar ranged as to admit of free mobility in various directions, so as to increase or diminish the cubic capacity of the thoracic cavity.
Of theDorsal Vertebrce . —That portion of the spine which enters into the composition of the thorax consists of the dorsal vertebrm, which are 12 in number, intermediate in size and position, between the cervical and the lumbar vertebrm. They form the main pillar of support for the whole respiratory apparatus —the great fixed point for the chief respira tory muscles to act against or draw upon. Their general appearance is that of increasing in size from above downwards ; but when carefully examined, they are as two cones, the apices of which touch at about the fourth or fifth vertebra, from which point, in either direction, they increase in their dimensions ; their breadth laterally exceeds their depth from before backwards. Their bodies are large and project deeply into the cavity of the tho rax, diminishing greatly the antero-posterior diameter of the chest. Out of twenty cases, the average projection is 2i of an inch, leaving little more than 4 inches for the heart and great blood-vessels.
A deep sulcus is thus formed, which if a cast be taken of the cavity of the thorax, is very striking. In phthisis pulmonalis, the space between the bodies of these verte brm and the sternum is sometimes less than one inch I When the thorax changes its form by disease, this centre pillar is liable to wedge in or jam up the thoracic organs against the walls of the chest.
The vertehrm are connected to each other by ligaments, and jointed beautifully into each other, so as collectively to admit of extensive motion, while there is hut little movement between any two vertebrm.
Of the bonds of union, the most remarkable are the inter-vertebral disks (ligaments inter vertebralia—Weitbr.)—composed of fibro•car tilage, and placed between the bodies of the vertebra, each disk serving to unite two vertebrm, and yet to permit a motion in any direction, yielding on that side towards which the column inclines, while on the contrary side it expands with the increasing intervertebral space. This substance is to the brain what
the cushion or " buffer " between each railway carriage is to the traveller ; it breaks a sud den jar from being transmitted from carriage to carriage. So does this intervertebral sub stance soften down any sudden jerk received at the lower extremity of the spine, preventing its being transmitted to the brain in the varied actions of walking, running, and leaping.
Of the Sternum (os pectoris : Xiphoides), so named from eripvov, the breast : is a kind of flattened bone, symmetrical in shape, which occupies the anterior and middle part of the thorax. It is supported by the ribs on either side ; it is broadest at its upper part, and then narrowed; it widens again, and finally becomes compressed and narrow where it joins the ensiform cartilage. (Fig. 660, e.) Its direction is oblique from above down wards and forwards. This, with the curva ture backwards of the spine opposite to it, increases the antero-posterior diameter of the thorax, as may be seen in a lateral view of a cast of the thoracic cavity.
The length of the sternum, which is pro portionably smaller in the female than the male, varies from 5 to 7i inches. At the upper part its breadth is from 14 to 2 inches.
Its thickness above is about 6 lines ; at its lowest part it is much thinner, never exceed ing three lines. The ancients compared the sternum to the sword of a gladiator ; and hence have arisen the denominations given to its various parts : as the handle (manubrium), the body (mucro), the point, or xiphoid appendix, (enszformis); but the last mention ed part now only retains the designation grounded on this circumstance. This division of the bone into three parts has been retained by some modern anatomists, who describe the three pieces of the sternum separately, as so many distinct bones; we shall only adhere to this in speaking of the development of the bone.