ANATOMY. 



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 unyielding 

 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 cynovia, 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 portionsof 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 joints. Joint 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 vertebrae, 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 cervical vertebrae to the occiput, and 



VOL. 1 



called the ligamentum muchx. In quad- 

 rupeds 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. 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 ot'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 umisual 

 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 basis, 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 

 manner, as that, when put together, the 

 hole 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 vertebrae from 



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