Although in all these instances there can be no doubt that the blood was in a poisonous state, there is no reason to suppose that this could have been foretold by any thing remark able in its appearance or sensible qualities. Scarcely more successful in general has been the search for extraneous poisons, which never theless have appeared from collateral circum stances to have entered the circulation, or have even been purposely introduced into it. Dr.Chris tisont has cited a sufficient number of cases where poisons swallowed have been afterwards found in the blood, to spew that we must not infer their absence from our inability in most cases to abstract them in a separate form ; and he further demonstrates how erroneous such an inference might be by stating that Dr. Coindet and himself, after destroying a dog in thirty seconds by injecting 82 grains of oxalic acid into the femoral vein, endeavoured in vain to detect any portion of it in the blood of the iliac vein and versa cava collected immediately after death, although it is highly improbable that it could have passed off by any of the secretions in so short a time.
The chief obstacles by which we are opposed in such researches are minuteness of quantity. and decomposition. When only a few grains of a poison are absorbed, and thence diffused not only through the whole mass of circulating blood, but likewise among all the various tissues and solids of the body, being moreover carried off by the kidneys, perhaps nearly as fast as they enter by the circulation, it cannot be matter of surprise, however delicate our tests may be, that they are seldom to be met with even where still retaining their chemical characters. When we consider, however, that reagents which produce a change of properties in those bodies with which they are brought in contact do probably themselves undergo a cor responding change, we shall readily perceive that our difficulties will be still further in creased on this account.
The products of diseased action, and espe cially pus, have been often met with, as well in the arteries and veins, as in the cavities of the heart ; but it yet remains a matter of doubt whether these are actually formed in the blood, or whether, as seems to me more probable, they are riot rather carried into the circulation from other parts in a degenerate or diseased state, or are the products of inflammation in the lining membrane of the bloodvessels them selves.
With respect to those cases where worms and insects are said to have appeared in the blood, whereof many are recorded, some are referrible to the head of false polypi, the shape of which has misled the observer, others to deception or the accidental presence of insects or their ova in the receiving vessel ; and though we cannot deny the possibility that parasitical animals may exist in the fluids as well as in the closed cavities and solids of the body, yet we require better evidence than has yet been ad duced to confirm our belief in the existence of entozoa in the circulating current. In a recent case brought forward by Mr. Bushnan,* and learnedly illustrated by that gentleman, it would, I confess, have carried more conviction to my mind, had he himself watched the blood from the moment of its quitting the vein until the larvae which he describes were seen swim ming in its serum. In such extraordinary cases the mind is not satisfied with anything short of moral certainty.
From what has been set forth in the fore going pages, it will be perceived that our knowledge on the subject discussed in them yet remains extremely defective. We learn, indeed, that under the existence of disease the different constituents of the blood are liable to morbid increase or diminution as well as to certain alterations in their sensible qua lities, hitherto less accurately examined ; that there are instances in which principles not usually met with in the healthy circulation may be detected in it, and others where those which are always present in a state of health do nearly if not altogether disappear. But that which
still remains unknown, and to which it is of the highest interest and importance that our investigations should be directed, is the con nexion that these morbid changes have with the diseases which they accompany ; the position which they occupy in the relation between cause and effect. Perhaps our present information is not sufficiently minute to give fair expecta tions of any considerable advances being made in this line of inquiry ; for when we contemplate the variety of materials for the formation and removal of morbid as well as of healthy secre tions and structures, which are stealing unper ceived along the vital current, we are forced to confess how small is the sum of all we know compared with that of which we are still igno rant, and how ample is the harvest which yet remains to be gathered by future labourers in this field of research.
BONE,(general anatomy in the normal state.) Gr. ocrrEoy. Lat. os. Fr. os. Germ. der Knochen. Ital. osso. The important offices fulfilled by bone in the animal economy, and its almost imperishable nature, could not but give it im portance in the eyes of the philosopher; whilst every language bears testimony to the high place it holds in popular estimation. We see it forming a framework to give shape and sup port to the body, cases and cages to protect the more delicate organs, levers by which loco motion is performed and force exerted. Again, we find it, among the tombs, successfully resist ing those destructive agents which a century before reduced the softer portions of the body to dust; and we speak of laying our bones in the grave as if they constituted the essential element of our frame.
We propose to treat of the general anatomy of bone under the following heads, viz.-1. its physical properties and intimate structure in man : '2. its periosteum and medulla, and its organization as a part of the living system : 3. its chemical composition : 4. its peculiarities in other animals.
1. The physical properties and intimate structure of bone in man.—The most remark able property in bone, and that which first arrests attention, is its extreme hardness com pared with other parts of the system. It is, indeed, the only substance in the body which deserves to be called hard ; all others are more or less soft, and are consequently destitute of that resistance and firmness by which bones are so admirably adapted for the offices they have to fulfil in the animal machine. The hardness usually increases with age. It varies a little in different situations, and depends, as we shall see, on the earthy matter which enters largely into the composition of the bones.
The colour of bone in the living person is a pale-rose colour, inclining, in early life to red, in old age to a yellowish white. Bones assume a beautiful white when macerated and deprived of the oily and sanguineous fluids which pervade them. The specific gravity of fresh bone is greater than that of any other animal substance. Bone is opaque or only slightly diaphanous. Bones are flexible and elastic. We find that the ribs may be bent and afterwards recover their form perfectly ; every schoolboy, indeed, knows the value of a horse's rib as a bow. This elasticity frequently saves them from fractures, and lessens the shock which would otherwise be communicated to the nervous centres and delicate structures they defend. It is possessed by every bone, and may be demonstrated in the oldest and most rigid by cutting them into thin slices.
Shape.—Bones assume every variety of shape, as might be expected from the use made of