The section devoted to the Articulations or Joints deals with the union of the various and dissimilar parts of the human skeleton. The followiing structures enter into the formation of joints. Bones constitute the basis of most joints. The long bones articulate by their ends, the flat by their edges, and the short at various parts on their surfaces. The articular ends are usually expanded, and are composed of cancellous tissue, surrounded by a dense and strong shell of compact tissue.
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Before describing the movements of the thorax as a whole, it must be premised that there are some few modifications in the movements of certain ribs resulting from their shape. Thus, the firs rib (and to a less extent the second also), which is flat on its upper and lower surfaces, revolves on a transverse axis drawn through the costo-vertebral and costo-transverse joints. During inspiration and expiration, the anterior extremities of the first pair of costal arches play up and down, the tubercles and the heads of the ribs acting in a hinge-like manner, the latter having also a slight screwing motion. By this movement, the anterior ends of the costal arches are simply raised or depressed, and the sternum pushed a little forward; it may be likened to the movement of a pump-handle.
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These articulations at the front of the thorax may be divided into four sets, viz.: The intersternal joints, or the union of the several parts of the sternum with one another. The costo -chondral joints, or the union of the ribs with their costal cartilages. The chondro-sternal joints, or the junction of the costal cartilages with the sternum. The interchondral joints, or the union of five costal cartilages (sixth, seventh, eighth, ninth, and tenth) with one another.
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These costo-vertebral articulations (French: articulations costo-vertébrales)consist of two sets: the capitular (costo-central): i.e., the articulation of the head of the rib with the vertebrae and the costo-transverse, or the articulation of the tubercle (of each of the first ten ribs) with the transverse process of the lower of the two vertebrae, with which the head of the rib articulates: i.e., the one bearing its own number, as the first rib with the first thoracic vertebra, the second rib with the second thoracic vertebra, and so on.
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There are two distinct sets of articulations in the vertebral column:
- Those between the bodies and intervertebral discs which form synchondroses and which are amphiarthrodial as regards movement.
- Those between the articular processes which form arthrodial joints.
The ligaments which unite the various parts may also be divided into two sets, viz. - immediate, or those that bind together parts which are in contact; and intermediate, or those that bind together parts which are not in contact.
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The Atlas is articulated with the occiput and the axis (Epistropheus).
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As in the intervertebral articulations, so in the union of the first portion of the sacrum with the last lumbar vertebra, there are two sets of joints -viz. (a) Class. -False Synchondrosis. (b) Class. -Diarthrosis. Subdivision. -Arihrodia (a) a synchondrosis, between the bodies and intervertebral disc; and (b) a pair of arthrodial joints, between the articular processes. The union is effected by the following ligaments, which are common to the vertebral column: -(i) anterior, and (ii) posterior longitudinal; (iii) lateral or short vertebral; (iv) capsular; (v) ligamenta flava; (vi) supraspinous and (vii) interspinous ligaments. Two special accessory ligaments on either side, viz., the sacro-lumbar and the ilio-lumbar, connect the pelvis with the fourth and fifth lumbar vertebrae.
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