LOCOMOTIVE ORGANS. THE JOIXTS. 157 



bone to the other, that is, from the pelvis to the head of the femur, 

 within the joint, offering an exceptional structure in the anatomy of 

 joints, and serving as an important check ligament, which is brought 

 into use in the act of standing. The hip-joint is very secure, and yet 

 the movements which it permits in the thigh, are very free; for the 

 thigh may be moved so as to make with the trunk the following angles: 

 forwards, 130; backwards, 40 to 60; outwards, 90; and inwards, 

 somewhat less. If the hip-joint be exposed, and the synovial capsule 

 be opened, the head of the bone remains in the socket, or acetabulum. 

 This is due, neither to the ligamentum teres, nor to the fibrous rim 

 which deepens the socket, but to atmospheric pressure; for, on making 

 a small aperture from the pelvic cavity through into the socket, air 

 enters, and the head of the bone falls out (Weber). The same thing 

 happens if the opened joint be suspended under the receiver of an air- 

 pump, and the air be then exhausted. 



There are certain forms of movable joint, which require special 

 description. The articulation between the upper ends of the radius 

 and ulna, sometimes named diarthrosis rotatorius, or lateral gingly- 

 mus, may be called a ring, or collar-joint ; for the side of the head of 

 the radius, convex in shape, is received into a little cuplike cavity on 

 the side of the ulna, from which a strong ligamentous ring or collar, 

 4, Fig. 54, passes completely round the head of the radius, tying it to 

 the ulna, and permitting the former bone to rotate round its long 

 axis, whilst resting upon the latter. The joint between the atlas and 

 the axis, is also somewhat similar in principle, but differs in its details : 

 thus the dentate process of the second vertebra, or axis, is received 

 into the anterior part of the ring of the atlas, and is held in position 

 by transverse and vertical bands of ligaments crossing behind it, 

 named the crucial ligaments; but the second vertebra is also con- 

 nected to the skull, for two check ligaments pass from the tip of the 

 tooth-like process, obliquely up to the occiput, and prevent the head 

 from rotating sideways beyond a certain point. The articulation 

 between the first vertebra and the occiput, is effected in reality by two 

 small gliding joints, the occipital condyles, which project downwards 

 on each side of the foramen magnum, being received into two concave 

 surfaces formed on the atlas. As there are here two separate joints, 

 one on each side of the median plane, the movement is limited to a 

 rocking motion, forwards and backwards. The nodding motion of the 

 head is accomplished between the cranium and first vertebra of the 

 neck; whilst the rotation of the head from side to side, carries the 

 atlas with it upon the axis ; the upper part of the spine may partici- 

 pate in both these movements. 



In certain other joints, plates of fibro-cartilaginous tissue are inter- 

 posed between the articular surfaces of the bones. Thus, in each of 

 the two articulations of the lower jaw, by means of its two condyles 

 with the very shallow glenoid fossae of the two temporal bones, there 

 is such an inter-articular cartilage; this is either very thin, or per- 

 forated, in its centre, but thick at its margins; it is perfectly movable, 

 though attached to the synovial membrane all round, and it follows 

 the movements of the jaw, so as to guard it from dislocation. The 



