154 SPECIAL PHYSIOLOGY. 



three different kinds, named the immovable, the mixed, and the mova- 

 ble joints. 



The immovable joints (synarthroses, from <rbv, $un, together, apdpov, 

 arthron, joint) include the several kinds of suture (sutura, a seam), 

 Fig. 8. The dentate or serrated sutures occur in the junction of the 

 bones of the upper part and sides of the cranium. In them the in- 

 dented or serrated edges of the adjacent bones fit into each other, 

 having, however, a thin layer of fibrous membrane passing between 

 them, which is derived from the periosteum and dura mater, and 

 serves not only to unite and nourish them, but to deaden shocks. The 

 suture is called squamous, where the adjacent margins of the bones 

 are bevelled off, so that one overlaps the other, as at the junction of 

 the temporal and parietal bones. Sometimes the direction of the 

 bevelling is changed at different parts of the same suture ; for, at the 

 upper part of the skull, the frontal bone overlaps the parietals, whilst 

 at the sides the parietals overlap the frontal an arrangement evi- 

 dently calculated to stiffen the tie between the two bones. Where the 

 borders of the adjacent bones are elevated, the suture is said to be 

 limbous (limbus, a selvage), as in the union of the parietal and occipi- 

 tal bones. In some sutures of the skull, as between the upper jaw- 

 bones, the palate bones, and others, the opposed margins are smooth 

 or even, and form a, false suture, named harmonia (fyetv, to fit). Some- 

 times a ridge in one bone is received into a groove in another (schin- 

 dylesis, ff%tv36fo)fftq), as in the junction of the vomer with the ethmoid. 

 The fitting of the teeth into their sockets, erroneously classed with the 

 joints, is called gomphosis (^o/^oc, gomphos, a nail). 



The mixed articulations (amphiartliroses, d.y.y\, together, or both, 

 apOpov, a joint) are those in which the opposed surfaces of the bones 

 are joined directly together by some intermediate soft substance, 

 which is fibrous externally, and more or less fibro-cartilaginous to- 

 wards its central part. The best examples of this joint are found in 

 those between the bodies of the movable vertebrae, from the second 

 downwards, Figs. 9, 10, 12, in the junction of the body of the last 

 vertebra with the sacrum, and in the articulation between the two 

 upper parts of the sternum. The symphysis pubis, or joint between 

 the two hip-bones at the front of the pelvis, and the joints between 

 the ilia and the sacrum behind, also possess the same general charac- 

 ters, but, in some respects, approach the more perfect or movable 

 articulations. The mechanical result obtained in these articulations 

 is great strength, accompanied by slight movement ; they serve, in all 

 cases, to deaden shock, and to give elasticity, and, in the case of the 

 movable vertebrae, they allow of limited motion, in all directions, be- 

 tween the numerous individual bones, the total result being consider- 

 able flexibility, and possibility of curvature, in the whole column. 



The movable joints (diarthroses, &, through, apOpov, a joint), so called 

 because the severance of the surfaces is complete, these not being 

 joined, but being merely in contact with each other, are the most per- 

 fect articulations in the animal economy. In them, the ends of the 

 bones are often expanded and variously shaped, according to the char- 

 acter of the joint, the end of one bone being usually convex and of the 



