Although the general form and outline of the joints is observable in the solid body of the bones, yet, as we have already stated, the cartilages contribute materially to the accurate completion of the parts, and the whole extent of articulating surface is covered with this body. Thus, a greater degree of motion is admitted than could have taken place if the joints had been entirely composed of a hard substance, and the bad effects of concussion are, in a con siderable degree, prevented. Still farther to facilitate motion by preventing friction, the joints are all inclos ed in a kind of bag, which is filled with a dense fluid, called synovia. To complete the mechanism of the joints, they are provided with a number of ligaments, which serve to keep the bones in their respective places, and to regulate their 'notions, so as to prevent their displacement except under circumstances of extraordinary violence: The bones of the ball and socket joint are united toge ther by a ligament which passes through their centres, while in the hinge joint the ligaments are placed at the sides of the bones ; but it would be encroaching on the province of the anatomist to enter upon a minute descrip tion of these parts. We shall only farther observe, that in none of the organs of the body is the adaptation of means to ends more remarkable than in the construction of the joints and the apparatus connected with them.
In its physical properties, bone is the most simple of any of the components of the body. Membrane, as we already remarked, is not possessed of any properties that are peculiar to the living system, and which do not be long to many other substances, but bone is'neithcr flexi ble nor extensible, and, except elasticity, has scarcely any physical properties which are not common to every kind of solid matter.
The mechanical structure of bone formed a part of the interesting investigations of Malpighi ; and it was dis covered by him that the basis consists of animal matter, arranged in the form of cells. Duhamel still farther ad vanced our knowledge on this subject ; but it appears to have been Herissant who first announced, that bone con tains earthy matter, and that upon this ingredient depend its hardness, and most of its peculiar properties. By means of diluted muriatic acid, we can dissolve the earth of the bones without acting upon the membranous cells ; if a bone, therefore, be immersed in this fluid, we shall deprive it, in a great measure, of its earthy matter, and reduce it to a soft, flexible, and elastic substance, retain ing its former shape and bulk, but having exchanged the properties of bone for those of membrane. In this pro cess, we have removed the earth and left the membrane ; but, to make the experiment complete, we must reverse the operation, and remove the membrane. This we ac complish by burning the bone, for in this operation it is the membrane alone which is consumed, the earthy mat ter being left untouched ; and if sufficient caution be used, it may be preserved without having its texture materially injured.
The membranous part of bone appears 'to be, like the other soft solids, essentially composed of fibres ; these, by their union, form horizontal plates, lying parallel to each other, and united together by smaller plates, cross ing the others at different angles, forming, by their inter section, the cells into which the earth is deposited. Some
writers have described the plates as held together by small processes, like nails ; but this seems to be a fanci ful conjecture, and is not supported by correct observa tion. Even the laminated structure of bone has been de nied by Bichat, who has endeavoured to prove that the facts which have been adduced in favour of its existence are all of them fallacious. To a certain extent, the obser vations and reasonings of Bichat appear to be correct ; and we think that there has been much fanciful conjecture exercised upon the subject, which probably had its ori gin from Duhamel, who thought that the substance of bone was composed of a succession of concentric layers, arranged like those which form the trunk of a tree. But, on the other hand, there appear to be many reasons for supposing that bones possess a proper plated texture ; of this, however, we shall have occasion to treat more fully hereafter.
Almost all bones, whatever be their shape, are the hard est on the external surface, and become less and less dense in their internal portion, until we arrive at a perfect ca vity, or at least, at a part where there are only a few plates, crossing in different directions, and leaving cells that are proportionally large. These have obtained the denomi nation of cancelli, or lattice-work. In the flat bones, both the external surfaces are hard, and the internal latice work is extended between the plates, presenting, in some measure, the same form with the bone itself. In the round bones, there is, of course, only one external surface, which is hard, like that of the flat bones, and a cellular cavity is left, which is more or less occupied by the cancelli, be sides which there is also another netWork of membrane, which does not contain earthy matter. It is on this mem brane that the marrow is lodged ; like the fat of the cel lular texture, it is contained in distinct cavities, which have no communication with each other.
One use of the internal cavities of bones is, by increas ing their external surface, to afford a greater space for the insertion of muscles and ligaments, and we accordingly find, that the ends of bones, to which the tendons are prin cipally fixed, possess most extent of surface. But this structure likewise serves the still more important purpose of increasing the mechanical strength of the bone, with out adding to its weight. The absolute strength of a bone may indeed be considered as depending upon the quantity of solid fibres which it contains ; but its power of resist ance may be mathematically demonstrated to be augment ed in proportion to its diameter, so that the same num ber of fibres placed, as it were, in the circumference of a circle, produces a bone more capable of resisting exter nal violence, than if they had been united in the centre, and the diameter of the bone had been proportionally di minished. Conformably to this principle, we find that the cylindrical bones, which possess the largest cavities, are in the extremities of the body, where, from their po sition, they are the most exposed to violence.