The Chest or Thorax consists of the twelve dor sal vertebral behind, of twenty - four ribs, twelve on each side, which spring from the ver tebrm behind and arch for wards, and of the breast-bone or sternum (o, p, q, Fig. 20), to which, by means of cartilages, most of the ribs are united in front. All the twelve ribs on each side (i to 12 in figure) are not connected with the breast-bone. The first seven have each a ' separate piece of gristle (cartilage—c, c) uniting them, and are therefore called "true ribs ;" the three next in succession are united by the same piece of cartilage, while the last two are quite unconnected with the sternum, and ter minate in the muscular wall of the belly. The hurt five are termed "false ribs," the last two (f, f) being also called "floating ribs." The attachment of the ribs to the vertebrae is effected by joints which allow of considerable movement, so that the ribs can be elevated or depressed, and thus the cavity which they inclose may be increased or diminished in size. The cartilages confer elasticity on the bony walls of the chest; but in advanced life much of this elasticity is lost by the cartilage becoming brittle owing to a deposit of lime in them, and consequently the chest becomes less capable of movement.
The arm is connected with the chest or thorax by means of two bones which together receive the name of the shoulder girdle. They are the collar-bone or clavicle (r, Fig. 20), and the shoulder-blade or scapula (s, Fig. 20). The collar-bone stretches front the top of the breast-bone (o, p, q) outwards, and forms the prominent point of the shoulder.
The scapula lies on the back of the thorax, where it is freely movable, and is attached to the outer end of the clavicle by strong fibrous bands. It has a strongly developed spine (e') for the attachment of muscles, and two processes (co•acoid s" and acrontion 8", the latter a prolonga tion of the spine) by which its at tachment to the clavicle is effected. At one of its angles it has a polished hollow (the glenoid cavity—t) lined with cartilage, into which the head of the arm-bone or humerus (Fig. 21) fits, thus forming the shoulder joint. The lower end of the hu merus is broadened out by a projec tion on both the outer and inner sides, the outer and inner condyles,and has a pulley-like surface (b) for articulating with the forearm to form the elbow joint. The forearm consists of two bones, ulna and radius. The ulna (Fig. 22,1) is the inner of the two, is large at the upper end, where it has )wa projections, one, the coro livid process (6), in front, and the other, the olecranon process (a), behind. Between these two is a deep groove (c) into which the smooth pulley-like surface of the humerus is received, forming a hinge joint. The olecranon behind forms the sharp prominence of the elbow. The lower end of the ulna is slender. The radius (Fig. 22, 2) is the outer of the two bones, is small at the upper end, where it is connected with the ulna, so as to permit its head to have a rotary move ment, and is large and ex panded at the lower end (y), where it forms part of the wrist joint. The arrangement
of these joints is such that the radius can roll, as it were, half-way over the ulna. It carries the hand with it, which is thus turned palm down wards--in the act of what is called pronation; when the radius is rolled back again the hand is turned palm up wards, that is, the back of the hand is now downwards---supination.
The wrist or carpus (Fig. 23) is made up of two rows of small bones, four in each mw. Beginning front the thumb side these bones are named as follows: Scaphoid (1), Semi-lunar (2), Cuneiform (3), and Pisiform (4), in the first row ; Trapezium (s), Trapezoid (e), Os Magnum (7), and Unciform (8), in the second row.
These small bones are allowed a slight amount of movement on one another by means of joints. The palm of the hand contains five shafted bones called meta carpal bones (m), which means coming after the carpus or wrist. These metacarpal' bones support the phalanges (p) or bones of the finger. There are three phalanges in each finger, diminishing in size towards the point. The thumb has only two phalanges (Fig. 23, p').
Like the upper extremity, the lower is con nected to the trunk by means of a girdle called the pelvis or pelvic girdle (Fig. 24). The pelvis is formed, behind by the prolongation of the vertebral column, os sacrum and coccyx, and at the sides by two large irregularly shaped bones, which curve forwards from the sides of the sacrum. The name given to each of these bones is innominate bone or the unnamed bone. In the figure the lines 1 on each side of the back-bone indicate the place of union with the sacrum. The innominate bones meet in front at the sym physis pubis (sp), a pad of gristle inter vening. Each innominate bone consists in early life of three pieces, termed Ilium (A), lschium (u), and Pubis (c). When the pelvis is looked at as a whole, a prominent ring can be followed from the sacrum behind round to the symphysis in hoot ; this is called the brim of the pelvis. The inclosed cavity contains the urinary and generative organs and the lower part of the bowel. In the erect position the pelvis forms an angle of 60° with the horizontal, so that the pres sure communicated by the back-bone is thrown, by this inclination, on to the innominate bones. Strong fibrous bands, passing from the sacrum to the part of the innominate bone called the ischium (at the point where the B is placed in the figure), prevent the sacrum from being forced forwards by the pressure. In the pubic portion of the bones is an irregular opening obturator foramen—(o) through which blood vessels, nerves, &c., pass outwards. In the innominate bone of each side is a large and deep irregularly formed cavity (a), which is lined with gristle and receives the head of the thigh-bone. It forms a very deep socket; and the thigh-bone has a correspondingly large globular head. This large socket is called the acetabulum.