Page:The American Cyclopædia (1879) Volume I.djvu/819

ARTICULATION 783 time brought into closer connection. The sym- metrical arrangement of the nerves suggests that of the members also ; and the limbs are found arranged in pairs, in the centipedes each pair proceeding from one of the articulations of the body. In the higher classes, as the crus- tacea, the same symmetry of pairs of limbs is perceived, and the connection of each pair with a segment of the body, even when the thorax, or body, needing no flexibility for loco- motive purposes, has its rings very obscurely defined. The lower groups contain the greater number of articulations or rings, and these are usually soft, upon an elongated body, furnished in most cases with no true limbs. Progressive motion is obtained by the bending of the flex- ible body in one and another direction, the muscles which effect this occupying a large portion of the body which in other animals is usually devoted more to the organs of nutrition and digestion. These in the articulata are not so elaborate as in the mollusca. The organs for respiration are much more highly organized, particularly in the insecta. In the air-breath- ing species the blood is aerated by being ex- posed to the action of the air introduced within the body, the fluid being distributed in cavities or tubes permeable to the air ; the former ap- pear to be analogous to lungs. In the articu- lata is found the greatest diversity of forms and habits of life. The largest animals of the divi- sion are the lobsters and crabs of the Crustacea ; the rest are, for the most part, of small size, many of them so minute as to pass unnoticed in the watery elements in which they abound. ARTICULATION, a term in anatomy, denoting the various modes of union between the bones of the skeleton. We may class articu- lations under three gen- eral heads, viz., mov- able joints, immovable joints, and joints of a mixed order, being somewhat movable, without much relative displacement of the contiguous surfaces. Movable joints are the most complex and va- rious in structure ; im- movable, the most sim- ple. Movable joints are common in the limbs, and the articulation of the lower jaw with the FIG. l. Elbow joint, show- skull; immovable joints Ing the hinge-like articu- _ oomrnon i n fl le head lation of the humerus with a the nlna. and face and lower por- l. Lower extremity of the tion of the trunk ; mixed npr3n. 0r forms of articulation 2, 8. upper extremity of th are common in the spi- utaa, or bone of the fore- nal column and the upper portions of the trunk. The hinge joints of the elbows and the knees, allowing free movements in one plane only, form one order of the movable class ; the ball-and-socket joints of the hip and shoulder, allowing free movements in a circular direc- tion, form a second order of the movable class ; and different combinations of these two orders, as seen in the articulations of the lower jaw with the skull, of the hands and feet with the arms and legs at the wrists and the ankles, and also of the bones of the hands and fingers, feet, and toes, form a third order of the movable class. The elbow joint, in fact, is of a com- pound order, being of the hinge-joint form with reference to the cubital movement of the forearm on the arm, and of the ball- and-socket form with reference to the radial movement of the forearm on the arm, in what are termed the supination and pronation of the hand and arm. The class of immovable joints may also be subdivided into different orders and varieties. In the sacrum and the pelvis many bones which are distinct at first literally grow together in some subjects, so as to efface all trace of original separation, while in others traces remain visible of former separation. In the cranium and the face there are numer- ous modes of junction between bones connect- ed by immovable articulation. The most prom- inent order of this class in the cranium is the serrated suture, the firmness of the union being increased by alternate notches or indentations and projections like the teeth of a saw formed on the edges of the bones, the teeth of the one being adapted to the indentations of the other. In this manner the bones of the skull unite at the top of the head and in the centre of the forehead. In other cases bevelled edges over- lap each other, and in this manner the tempo- ral bones are joined to the parietal bones of the skull. Another form of fixed articulation is the ridge-and-groove, a ridge being formed on the edge of one bone and a grooved fissure in another to receive it. By this means the bony part of the septum of the nose is inserted into the floor of the nasal cavity to divide the nostrils, and thus form a double cavity by means of a partition wall. The mixed class of articulations contains many varieties of adapta- tion. The mode in which ribs are attached to the spinal column behind and to the sternum in front forms one simple order of the mixed class; the mode in which the vertebra are connected with each other in the spinal col- umn, another, more complex ; and the mode in which the slightly yielding portions of the pel- vic articulations are connected, a third and simple order of this class. The movable artic- ulations, being the most complex in form and structure, will give the best idea of the various elements of an articulation ; and the ball-and- socket joint, being the most simple of this kind, will serve the purpose of a simple illustration. In the hip joint we have a kind of ball, or rounded surface, at the head of the thigh bone, which hemispherical surface is capped with a thin layer of cartilage, somewhat elastic in structure, and exceedingly smooth on its exter- nal surface. In the bones of the pelvis a socket