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- THE CHEST, PATELLA, AND SHOULDER-BLADE.
Fig. 1. The spine, ribs, and sternum, constitute the frame work of the chest or thorax. Referring, however, to the plate, or to nature, we observe that the ribs are not continued throughout from the spine to the sternum, but intervening cartiluges complete the form of the chest, by connecting the end of the first ten ribs to the breast bone. This is a farther provision, relative to the mechanical function of the lungs, deserving notice. The muscles of respiration enlarge the capacity of the chest by elevating the ribs; and during the momentary interval of muscular action, the cartilages, from their great elasticity, restore the ribs to their former position.
Fig. 2. Represents the true shape of the patella, the anterior surface convex. Fig. 3, the posterior surface, which has two concave depressions adapted to the condyles of the thigh bone. The projection of the patella, as a lever, or pulley, removes the acting force from the centre of motion, by which means the muscles have a greater advantage in extending the leg. That this bone is “ unlike any other in the body,” is a mistake; such bones are numerous, though less obvious, for they do not exceed the size of a pea: these are called sesamoid bones, and are formed in the flexor tendons of the thumb, and sometimes in the fingers. They are frequently found under the tendons of some of the muscles. Two of these sort of bones are constantly found under the articulation of the great toe with the foot: some also are discovered, though not so constantly, under the corresponding joints of the other toes. The sesamoid bones, like the patella, remove their tendons from the centre of motion, facilitate their glidings over the bone, and protect their articulations.
FIG. 4. The shoulder-blade (scapula) is joined to the collar bone oy ligaments, and to the thorax by powerful muscles which are capable of sustaining immense weights, and wnose action gives the various directions to the arm, and enables it freely to revolve at the shoulder-joint.
Fig. 5. The os hyoides, a small bone situated at the root of the tongue. It serves as a lever or point for attaching the muscles of the tongue, larynx, and those of deglutition.
PLATE XI.- THE HIP, KNEE AND ANKLE JOINTS.
FG. 1. The capsular ligament is here opened in order to show the ligament of the hip, named the round ligament. It allows considerable latitude of motion, at the same time that it is the great safeguard against dislocation.
Fig. 2 and 4. The crucial or internal ligaments of the knee-joint arise from each side of the depression between the condyles of the thigh-bone; the anterior is fixed into the centre, the posterior into the back of the articulation of the tibia. This structure properly limits the motions of the joint, and gives the firmness requisite for violent exertions. Viewing the form of the bones, we should consider it one of the weakest and most superficial, but the strength of its ligaments and the tendons passing over it, render it the most secure, and the least liable to dislocation of any joint in the whole body.
Fig. 3. One of the interarticular cartilages of the knee, from their shape called semilunar; it is also represented in situ, Fig. 2. The outer edge of each cartilage is thick, the inner concave edge thin; the sockets for the condyles of the thigh-bone are thus rendered deeper, and the cartilages are so fixed as to allow a little play on the tibia, by which the joint moves with great freedom.
A moving cartilage is not common, but is peculiar to those joints whose motions are very frequent, or which nove under a great weight. It is a contrivance found at the jaw-bone, the inner head of the collar-bone and the articulation of the wrist, as well as at the knee. The obvious use is to lessen friction and facilitate motion.
Fig. 4. a, the fibula ; b, the tibia, the lower extremities of which, c, d, form the outer and inner ankle, and receive, e, the great articulating bone of the foot, called the astragalus between them. When the foot sustains the weight of the body the joint is firm, but when raised it easily rolls on the ends of these bones, so that the toe is directed to the place on which we intend to step.