158 SPECIAL PHYSIOLOGY. 



double character of the articulation of the lower jaw with the temporal 

 bones at the base of the cranium, also necessitates a hinge-like action 

 of this upon the skull ; at the same time, other slighter, Literal, and 

 backward and forward, movements are permitted. Again, between 

 the inner end of the clavicle and the sternum, there is also an inter- 

 articular fibro-cartilage, which passes obliquely from the upper border 

 of the collar-bone to the cartilage of the first rib, close to the lower 

 border of the socket in the sternum. This direction is the one best 

 fitted to resist thrusts or shocks coming from the shoulder, which must 

 constantly take place in the action of the upper limb. Passing side- 

 ways from the lower end of the radius to the lower end of the ulna, is 

 another fibro-cartilage, 8, Fig. 54, which ties those bones together, 

 whilst it permits the radius, which is marked on its inner side by a 

 slight concavity, to roll or rotate on a corresponding convexity upon 

 the ulna. Lastly, in the knee-joint, are two remarkable fibro-carti- 

 lages, having a more or less crescentic form, and hence called the 

 semilunar cartilages. They rest on the upper end of the tibia, and 

 present their convex thick borders towards the outer and inner sides 

 of the joint respectively, where they are attached to the synovial mem- 

 brane and ligaments, whilst their thin concave borders are turned 

 towards each other, i. e., towards the centre of the joint ; they serve 

 to deepen the two shallow sockets on the head of the tibia, into which 

 the condyles of the femur are received. 



Elastic ligaments. Between the arches of all the movable vertebrae, 

 with the exception of the atlas and axis, also between the arch of the 

 last lumbar vertebra and the corresponding part of the sacrum, very 

 peculiar ligaments are found, differing in nature from the white fibrous 

 ligaments which, as their oifice requires, are essentially non-extensible. 

 These peculiar ligaments, the lig amenta subflava, are composed of yel- 

 low elastic tissue, and are highly extensible and elastic ; they not only 

 serve to connect the bones, but exercise a special mechanical office, 

 yielding, for example, to permit of the slight separation of the verte- 

 bral arches from each other in the forward bending of the spine, and 

 assisting mechanically, by their elastic recoil, in the re-erection of the 

 body, and in its due maintenance in an upright posture, thus counter- 

 acting the effect of the muscles which would flex the spine. The yel- 

 low elastic tissue, indeed, is used here, as elsewhere, in the economy, 

 to sustain weight or any force, or to overcome constant resistance, 

 without the expenditure of muscular power. 



The tendons, fascice, and sheaths of the muscles. The tendons of 

 the muscles, as we shall immediately see, serve to convey the muscular 

 force from, and to, definite parts of the skeleton, and should therefore 

 be regarded amongst the passive organs of locomotion ; they have 

 merely mechanical functions, whether they be considered as adjuncts 

 to the muscles, or as contributing to the support of the joints over 

 which they pass. The fascice, too, are strengthened in certain parts, 

 and are so arranged as to inclose, or bind down, subjacent muscles, 

 generally or individually, in special sheaths, and so prevent them di- 

 verging from their required lines of action ; and it is certain that they 

 aid in muscular efforts, by holding and supporting the muscles. They 



