CHAPTER XIV.

ON THE ATTITUDES AND MOVEMENTS OF MAN.

In explaining the functions of the heart and lungs, the disposition of the bones of the chest was described, and on the same occasion and on others, I have had to advert to the action of different classes of voluntary muscles. We have now to consider the entire frame of the skeleton in reference to the postures and movements of the body, and the action of the voluntary muscles of the trunk and limbs. Let us begin with an examination of the structure and chemical composition of bone.

Upon making sections of a dry bone, we find it composed of two parts; externally, of a compact crust of greater or less thickness, and internally, of a series of delicate plates and processes that intercept innumerable small cells or cancelli, which freely communicate.

The bones of the skeleton affect three principal forms; each of which has some peculiarity in its structure adapted to the object upon which it is employed.

The flat bones are those which belong to the great visceral cavities, the cranium, the chest, the pelvis. In these bones the outer crust is thin, and forms what are termed tables, the outer and the inner: the interposed cancellated structure is termed the diploe. The two tables of the flat bones are for the most part parallel. In the skull the inner table is of a closer grain and of greater density than the outer. The external surface of flat bones is generally convex, a circumstance which

Of the different Kinds of Bones.

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contributes with the alternating compactness and porousness of their texture to protect the parts within against any kind of injury.

The round or cuboïd bones are small irregular cubes or portions of cylinders, one series of which forms the vertebral column, another the wrist, and a third the instep: their crust is yet thinner than that of the flat bones; their internal structure varies in different instances: the cancelli are fine in the vertebræ; coarse in the tarsal and carpal bones. The texture of cuboïd bones is therefore any thing but brittle, and is well calculated to deaden the force of jars and concussions of all sorts. To promote the latter object, the cuboïd bones are found not separate but in groups, so that the elasticity, resulting from many joints and intervening layers of cartilage, gives additional effect to their structure. Another advantage results from forming the parts described of many bones; a considerable latitude of motion may thus exist in the entire part, and at the same time no single joint have play enough to risk its security.

The long or cylindrical bones are employed as levers upon which the muscles act in supporting and impelling the body. The extremities of a cylindrical bone, where it is articulated to those adjoining, as they have the office, so likewise have they the structure of the cuboïd bones, a thin outer crust, and strong cancelli: they likewise generally assume a considerable breadth, which increases the strength of the joints.

But the intermediate part or shaft of a long bone is contrived differently; its crust is of great thickness, from to of an inch; while the plates belonging to its cancelli are remarkably fine and delicate. The bony matter, spread

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Chemical Composition of Bone.

out in thin and separate plates in the extremities, seems collected in the shaft to form a compact cylinder, in order that the lever which it represents may not be flexible; and the cylinder is hollow, to give the greatest strength to a determinate weight of bony substance.

If a bone be calcined, the earth which remains has the same form and structure as before; but it is rendered brittle, and falls in pieces almost from its own weight. If a bone be steeped in acid, it retains its form and structure, but becomes perfectly flexible.

The following table exhibits the composition of calcined human bones according to the analysis of Berzelius.

1

Recent bones are covered with a membrane termed their external periosteum, which is easily detached from their surface; it is thin, except where tendons or ligaments are inserted. All the cavities in a bone again are lined with a fine membrane termed the internal periosteum; from its surface is secreted the marrow or animal oil which fills the cancelli. Upon examining the bones in a favourable subject minutely injected with size and vermilion, blood-vessels may be traced through their entire substance. Neither lymphatics nor nerves have been followed into bone; but absorption evidently takes place during the growth of bone, and if a young animal

be fed with madder, in six weeks time red earthy matter seems substituted in the place of white earth in all its bones. During health bones do not appear sensible to any stimulus: during disease they exhibit acute sensibility.

The modes, in which the bones of the skeleton are joined together, are very various; in some instances no motion is allowed between adjoining bones, and they seem to have been left disunited with the object only of diminishing the effect of concussion. The bones of the head are thus disunited or united through the intervention of membranous substance only. In parts of the cranium where strength is required, the bones are dovetailed together, and the joint is called a suture: in other instances the bones meet at an even line, which is termed union by harmonia, or if a process of one bone is received into a corresponding cavity in another, the juncture is termed schindylesis, or gomphosis.

In other instances, where no motion is intended to take place, but where a part has often to resist considerable violence, a portion of white elastic substance termed fibrous cartilage is interposed between two bones, with the extremities of either of which it is continuous; in this manner the ossa innominata and sacrum are joined together. As a variety in this sort of articulation we may remark that the true ribs are joined to the sternum by portions of fibrous cartilage, which are received into sockets at the side of the breast-bone, but are not continuous with it, if we except the first: a layer of membrane is interposed between the cartilages of the other ribs and the breast-bone, so as to allow of the requisite motion at the sterno-costal joints during the dilatation of the chest.

In the kind of joint last described another substance called a ligament is generally found in addition. Ligaments are white silvery bands composed of very delicate fibres, that, where they are flexible, have but little elasticity except in a few instances: they are composed nearly wholly of gelatin; they have little sensibility to common stimuli, but when stretched feel acute pain.

The junction of the bodies of the vertebræ deserves to be particularly described. In fish, in which the spine is very flexible, the articular surfaces of the bodies of the vertebræ are so excavated, that when two meet they inclose a cavity the shape of which may be called spherical: this cavity is filled with fluid, which we will suppose to be incompressible, and the margins of the two vertebræ are joined together by the intervention of ligamentous substance, which is highly elastic; thus a double ball and socket joint exists between every two vertebræ, each of which is capable of rolling in every sense upon the ball of liquid contained between the two. In the human spine the same type is followed, but with a provision for much less latitude of motion; the excavation is shallow, the central substance semifluid, and the surrounding fibrous cartilage is confined by ligamentous bands of less elastic substance.

In the more elaborate joints two other elements are met with. The articular extremities of the bones are tipped with cartilage, and a fine membrane is reflected over its surface and over that of the capsular ligament by which the bones are joined together. Membranes of this description in many respects resemble serous membranes: they form shut sacs of the finest texture, and can be separated though not without difficulty from the