95. "In the most extensive motion of which the spinal column is capable, that of flexion, the common anterior ligament is relaxed; the fore part of the intervertebral substance is compressed, and its back part stretched; while the common posterior ligament is in a state of extension. In the extension of the column the state of the ligaments is reversed; those which were extended being in their turn relaxed, while the common anterior vertebral is now put upon the stretch. In the lateral inclination of the column, the intervertebral substance is compressed on that side to which the body is bent. In the rotatory motion of the column, which is very limited in all the vertebræ, but more particularly in the dorsal, in consequence of their attachment to the ribs, the intervertebral substance is contorted, as are likewise all the ligaments. All the motions of the column are capable of being aided to a great extent by the motion of the pelvis upon the thighs."
96. "The number and breadth of the attachments of these bones," says an accomplished anatomist and surgeon,[4] "their firm union by ligament, the strength of their muscles, the very inconsiderable degree of motion which exists between any two of them, and lastly, the obliquity of their articular processes, especially in the dorsal and lumbar vertebræ, render dislocation of them, at least in those regions, impossible without fracture; and I much doubt whether dislocation even of the cervical vertebræ ever occurs without fracture, either through their bodies or their articular processes. The effects of each of these accidents would produce precisely the same injury to the spinal marrow, and symptoms of greater or less importance, according to the part of the spinal column that is injured. Death is the immediate consequence if the injury be above the third cervical vertebra, the necessary paralysis of the parts to which the phrenic and intercostal nerves are distributed causing respiration instantly to cease. If the injury be sustained below the fourth cervical vertebra, the diaphragm is still capable of action, and dissolution is protracted. The symptoms, in fact, are less violent in proportion as the injury to the spinal marrow is further removed from the brain; but death is the inevitable consequence, and that in every case at no very distant period."
97. So the object of the construction of the spinal column being to combine extent and freedom of motion with strength, and it being necessary to the accomplishment of this object to build up the column of separate pieces of bone, the connecting substances by which the different bones are united are constituted and disposed in such a manner as to prove absolutely stronger than the bones themselves. Such is the structure of this important portion of the human body considered as a piece of mere mechanism; but our conception of its beauty and perfection would be most inadequate if we did not bear in mind, that while the spinal column performs offices so varied and apparently so incompatible, it forms an integrant portion of a living machine: it is itself alive: every instant, blood-vessels, absorbents and nerves, are nourishing, removing, renewing, and animating every part and particle of it.
98. The anterior boundary of the thorax is formed by the bone called the sternum, or the breast-bone, which is broad and thick at its upper, and thin and elongated at its lower extremity (figs. XLV. 6, and LIV.), where it gives attachment to a cartilaginous appendix, which being pointed and somewhat like a broadsword, is called the ensiform cartilage.
Anterior view of the sternum.
99. Its position is oblique, being near the vertebral column at the top, and distant from it at the bottom (fig. XLV. 6). Its margins are thick, and marked by seven depressions, for the reception of the cartilages of the seven true ribs (fig. LIV). Its anterior surface is immediately subjacent to the skin, and gives attachment to powerful muscles, which act on the superior extremities: its posterior surface is slightly hollowed in order to enlarge the cavity of the thorax (fig. LV.).
Posterior view of the sternum.