Consfruction a/ a joint—The opposed surfat.es of hones, whiell form joints, arc covered by a thin crust of cartilage, most exquisitely smooth and polished. Hence they move on each other, in whatever di. rection their structure admits, without any hindrance from friction. They are tied together by strong and unyieldirig cords, resembling tendons, and known by the name of ligaments. These keep the surfaces of the bones together, and re strict their motions to certain directions. In order still further to promote the fa cility of motion, and to obviate every pos sibility of friction, the cartilaginous sur faces are smeared with an unctuous fluid, called .svnovia, which makes them per fectly slippery. This fluid is confined to the surface of the joint by means of a thin and delicate membrane, called the cap sular ligament, which envelopes the joint. It is secreted from portions of a fatty sub stance, called the synovial glands. The ligaments are usually situated on the out side of the capsula ; but in many instances they are contained in the cavity of the joint, passing from the centre of one bone to another. These are called interarticu lar ligaments.
Particular of the lower jaw. This is formed between the con. dyle of the jaw and a hollow in the tem poral bone. It contains a moveable car tilage, which renders the articulation more secure, when the jaw is brought forwards on the bone under certain cir cumstances.
The connection of the head to the ver tebrae is effected by means of two promi nences of the occiput, which are received into corresponding cavities of the atlas. By this joint the nodding motions of the head are performed. But the atlas itself turns horizontally round the tooth-like process of the vertebra dentata, and as the head is closely connected to the atlas, it is carried round at the same time. Therefore, the lateral or rotatory motions of the head are performed by a different joint from that which performs the nod ding motions. Neither of these articula tions admits of very extensive motion ; but the deficiency is compensated by the mobility of the vertebra,. which enable us to carry the head freely in any direc tion we may wish. The head rests near ly in equilibrio on the spinal column; yet, if left to itself, it would fall forwards, as the joint is not precisely in the centre of the 'basis cranii. To counteract this ten dency, there is a ligamentous substance extended from the spinous processes of the nervirul velgehi,- in ihn nrritlelt nnr1 called the ligamenturn muchx. In quad rtipeds this can be best seen, as the weight of the head is there supported to a'much greater disadvantage. The mus cles also contribute to keep the head up right; and hence, when a man drops asleep sitting, the relaxation of the ex tensor muscles causes the head to nod forwards.
Joints of the spine, or back bone, is a chain of joints of very wonder ful construction. Various, difficult, and almost inconsistent, offices were to be ex ecuted by the same instrument. It was to be firm, yet flexible ; firm, to support the erect position of the body; flexible, to allow of the bending of the trunk in all degrees of curvature. It was further,
also, to become a pipe or conduit for the safe conveyance of a most important part of the animal frame, the spinal marrow ; a substance, not only of the first necessity to action, if not to life, but of a nature so delicate and tender, so susceptible, and so impatient of injury, as that any unusual pressure upon it, or any considerable ob struction of its course, is followed by paralysis or death. It was also to afford a fulcrum, stay, or basis, for the insertion of the muscles which are spread over the trunk of the body, in which trunk there are not, as in the limbs, cylindrical bones, to which they can be fastened ; and like wise, which is a similar use, to furnish a support for the ends of the ribs to rest upon.
The breadth of the bases, upon which the parts severally rest, and the closeness of the junction, give to the chain its firm ness and stability; the number of parts, and consequent frequency of joints, its flexibility; which flexibility, we may also observe, varies in different parts of the chain ; is least in the back, where strength more than flexure is wanted ; greater in the loins, which it was necessary should be more supple than the back ; and great est of all in the neck, for the free motion of the head. Then, secondly, in order to afford a passage for the descent of the medullary substance, each of these bones is bored through in the middle in such a mannr, as that, when put together, the bolt in one bone falls into a line and cor responds with the holes in the two bones contiguous to it; by which means the perforated pieces, when joined, form an entire, close, uninterrupted channel. But, as a settled posture is inconsistent with its use, a great difficulty still- remained, which was, to prevent the vertebra from shifting upon one another, so ns to break the line of the canal as often as the body moves or twists, or the joints gaping ex ternally, whenever thc body is bent for wards, and the spine thereupon made to take the form of a bow. These dangers, which are mechanical, are mechanical ly provided against. The vertebra, by means of their processes and projections, and of the articulations which some of these form with one another at their ex tremities, are so locked in and confined, as to maintain, in what are called the bo dies or broad surfaces of the bones, the relative position nearly unaltered; and to throw the change and the pressure pro duced by flexion almost entirely upon the intervening cartilages, the springiness and yielding nature of whose substance admits of all the motion which is necessa ry to be performed upon them, without any chasm being produced by a separa tion of the parts. I say of all the motion which .is necessary; for, although we bend our backs to every degree almost of inclination, the motion of each vertebra is very small : sucli is the advantage which we receive from the chain being composed of so many links. Had it been composed of three or four bones only, in bending the body the spinal marrow must have been bruised at every angle.