ANATOMY. 



t cartilage, rno-,t 



exquisite!) smooth ami polished. Hence 

 they mo\i on caeh other, in whatever di- 

 rection their structure admits, without 

 any hindrance from friction. They arc- 

 tied tog-ether by strong and unyielding 

 cords, resembling tendons, and known by 

 the name of ligaments. These keep the 

 surfaet s 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 synovia, 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- 

 ce, 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. 



J'.n-ticiilar joints. Joint of the lower 

 jaw. This is formed between the con- 

 ch le 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- 

 tebra- is efle< ted by means of two promi- 

 nences of the occiput, which are received 

 into corresponding cavities of the atlas. 

 H\ this joint the nodding motions of the 

 Lead arc- performed. But the atlas itself 

 turns horizontally round the tooth-like 

 process of the \ ertcbra dentata, and as 

 the head is closely connected to the atlas, 

 it is carried round at the same time. 

 Therefore, tlu- lateral or rotatory motions 

 of the luad are performed by a different 

 joint from that which performs tin- nod- 

 motions. Neither of these articula- 

 tions admits of very extenshe motion; 

 but the deficiency is compensated b\ 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 cquilihrio on the spinal column; yet, 

 it lift to itself, it \\ould 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 vertebra: to the occiput, and 



\OL. i. 



(ailed thf ligamentum much*. In quad- 

 rupeds this can be best seen, 

 weight of tin head is there supported to 

 a much greater disadvantage. The mus- 

 <-K s also contribute to keep the head up- 

 right ; and hence, vrhcn a man drops 

 asleep sitting, the relaxation of the ex- 

 tensor muscles causes the head to nod 

 forwards. 



Jiiint.t of t fie 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 ol 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 fi-ame, 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, jn 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 aU.. 

 observe, varies in different parts of thr 

 chain ; is least in the back, where strength 

 more than flexure is wanted ; gn 

 the loins, which it was necessary should 

 be more supple tAan 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 borert through in the middle in such a 

 i , as that, when put together, the 

 holi 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 



\ i 



