minate the fibrinous portion of the liquor sanguinis itself, when it has coagulated on the surface, or in the substance of any tissue or organ of the body. This term meets a difficulty which morbid anatomists have long experienced and hence it has of late years been extensively used to signify various kinds of morbid deposits. Thus it has been applied to all those processes hitherto termed inflammatory, tubercular, and cancerous; to all kinds of tumour and morbid growth, and what has been called melanosis and typhous deposit. It is often the cause of many concretions, and frequently constitutes the soil for parasitic vegetations or cryptogamic plants of a low type, which communicate essential characters to certain diseases. Indeed exudation, as a morbid process, comprises the greater part of organic, as distinguished from functional diseases; of lesions of nutrition, as separated from lesions of innervation.

I. EARLY PHENOMENA OF EXUDATION.-Exudation in every case results from a previous series of changes which has taken place in the capillary vessels, and blood contained in them. These changes, as we are enabled to follow them in the transparent parts of animals under the microscope, are seen to occur in the following order :-1st, The capillary vessels are narrowed, and the blood flows through them with greater rapidity. 2d, The same vessels become enlarged, and the current of blood is slower, although even. 3d, The flow of blood becomes irregular. 4th, All motion of the blood ceases, and the vessel appears fully distended. 5th and lastly, The liquor sanguinis is exuded through the walls of the vessel, sometimes accompanied by extravasation of blood corpuscles, owing to rupture of the capillaries.

The first step in the process, viz., narrowing of the capillaries, is readily demonstrated on the application of acetic acid to the web of the frog's foot. If the acid be weak, the capillary contraction occurs more slowly and gradually. If it be very concentrated, the phenomenon is not observed, or it passes so quickly into complete stoppage of blood, as to be imperceptible. Although we cannot see these changes in man under the microscope, certain appearances indicate that the same phenomena occur. The operations of the mind, for instance, as fear and fright, and the application of cold, produce paleness of the skin; an effect which can only arise from contraction of the capillaries, and a diminution of the quantity of blood they contain. In the majority of instances, also, this paleness is succeeded by increased redness, the same result as follows from direct experiment on the web of the frog's foot, constituting the second step of the process. In other cases, the redness may arise primarily from certain mental emotions, or from the application of heat. In either case it depends on the enlargement of the capillaries, and the greater quantity of blood they

contain.

The variation in the size of, and amount of blood in, the capillaries, is conjoined with changes in the movement of that fluid. Whilst the vessels are contracted, the blood may be seen to be flowing with increased velocity. After a time the blood flows more and more slowly, without, however, the vessel being obstructed: it then oscillates, that is, moves forwards and backwards, or makes a pause, evidently synchronous with the ventricular diastole of the heart. At length the vessel appears quite distended with yellow corpuscles, and all movement ceases.

Again, these changes in the movement of the blood induce variations in the relation which the blood corpuscles bear to each other, and to the walls of the vessel. In the natural circulation of the frog's foot, the yellow corpuscle may be seen rolling forward in the centre of the tube, whilst on each side a clear space is left, only filled with liquor sanguinis, and a few lymph corpuscles. There are evidently two currents, the centre one very rapid, that at the sides (in the lymph spaces, as they are called), very slow. The coloured corpuscles are hurried forward in the first, occasionally mixed with some lymph corpuscles. These latter, however, may frequently be seen clinging to the sides of the vessel, or slowly proceeding a short distance down the tube in the lymph

space, and then again stopping. Occasionally they get into the central torrent, when they start off with great velocity, and accompany the yellow corpuscles. It has been said that these corpuscles augment in number, accumulate in the lymph spaces, and obstruct the flow of blood. In young frogs their number is often very great; but then they constitute a normal part of the blood, and in no way impede the circulation. In old frogs, on the other hand, all these, and subsequent changes, may be observed, without the presence of colourless corpuscles. When the capillaries enlarge, however, the central coloured column in the smaller vessels may be seen to enlarge also, and gradually approach the sides of the tube, thus encroaching on the lymph spaces. The slower the motion of the blood, the closer it comes, until at length the coloured corpuscles come in contact with the sides of the vessel, and are more or less compressed and changed in form. At length the vessel is completely distended with coloured corpuscles, the original form of which can no longer be discovered, and the tube appears to be filled with a homogeneous deep crimson fluid. This is congestion. If the morbid process continue, the vessel may burst, causing hemorrhage, or the serum and liquor sanguinis may transude through its walls, without rupture, into the surrounding texture. This is exudation.

PORTION OF WEB OF THE FROG'S FOOT, VIEWED WITH A MAGNIFYING POWER OF 250 DIAMETERS — On the left of the figure the circulation is natural; in its middle portion the column of blood is oscillating, and the corpuscles crowded together; on the right the circulation has stopped, and exudation has taken place. About the centre hemorrhage has occurred, owing to laceration of a capillary vessel.

II. THEORY OF EXUDATION.-It is of the utmost importance in pathological inquiries to separate facts from theories. Our facts may be correct, although the conclusions derived from them are wrong. This proposition, however generally admitted, is seldom acted on in medical inquiries, in which we find fact and hypothesis so mingled together, that it often requires considerable critical and analytical power to separate one from the other. We are, however, in all cases, insensibly led to theorise-that is, to attempt an explanation of the phenomena observed, in order that we may derive from them some general principle for our guidance. Such speculation is always legitimate, so long as we consider opinions to be mere generalisations of known facts, and are ready to abandon them the moment other facts point them out to be erroneous. The phenomena of exudation, previously described, may easily be demonstratedthey constitute the facts. Let us now examine how they have been attempted to be explained-in other words, what is the theory.

1. The contraction and dilatation of the capillaries is explicable, by supposing them to be endowed with a power of contractility analogous to that

existing in non-voluntary muscles. John Hunter thought they were muscular, from the results of his observations and experiments; and they may be shown by the histologist to consist of a delicate membrane, in which permanent nuclei are imbedded. In structure, then, they closely resemble the muscular fibres of the intestine, and we know that, like them, they may be contracted or dilated by emotions of the mind, or by local applications. The narrowing of these tubes, therefore, may be considered, as Cullen thought it was, analogous to spasm, while their dilatation is similar either to the relaxation which follows such spasm, or to muscular paralysis.

2. The rapid and slow movement of the blood is explicable on the hydraulic principle, that when a certain quantity of fluid is driven forward with a certain force through a tube, and the tube is narrowed or widened, while the propelling force remains the same, the fluid must necessarily flow quicker in the first case and slower in the second. It has been supposed, from the throbbing of large vessels leading to congested parts, that they pump a larger quantity of blood than usual into them. This was called "determination of blood," by the older pathologists, but is now known not to be a cause, but a result, of the changes going on in the capillary vessels. The oscillatory movement, seen in the transparent parts of small animals, has not been seen to exist in man, and probably depends, in the former, on a weakened power of the heart.

3. It is the stoppage of the blood, and exudation of the liquor sanguinis, however, which it is most difficult to explain; for why, so long as there is no mechanical obstruction (and during this process none has ever been seen) should the circulation through the capillaries of a part cease? It has been endeavoured, indeed, of late years, to establish a mechanical obstruction, by supposing the formation of colourless corpuscles, in large numbers, which cling to the sides of the capillaries, and so cause interruption of the stream. But this hypothesis is negatived by the following facts:-1st, In young frogs the vessels may be seen to be crowded with colourless corpuscles, while the circulation is in no way affected. 2d, In old frogs, oscillation and gradual stoppage of the stream may be seen, without any colourless corpuscles being present. 3d, The colourless corpuscles, as shown by Remak, are increased, after large venesections, in the horse, without ever causing active congestion. And 4th, Cases have occurred in man, where all the vessels have been crowded with colourless corpuscles, associated with hypertrophied liver and spleen, and yet no active congestion in these vessels, nor exudation of any kind, has been occasioned.

We cannot ascribe the stoppage of the circulation in the capillaries to venous obstruction, or to mechanical pressure of any kind, because all observation proves that such causes, while they induce effusion of serum, never occasion exudation of liquor sanguinis. We are compelled, therefore, to ascribe the vital force producing these changes, not to anything residing in the blood, or in the vessels, but to the tissues which lie outside the vessels. Whether we give to this force the name of attraction, or whether we consider it a modification of the power which, in a state of health, attracts nutritive materials from the blood, is of little consequence-such seems to be the only active agency to which we can ascribe the approach of the coloured particles to the capillary walls, and the passage through them of the exudation.

III. RESULTING PHENOMENA OF EXUDATION.-When the liquor sanguinis is exuded, it generally coagulates, and constitutes a foreign body in the texture of the parts affected, which it becomes the object of nature either to remove from the system, or so to modify that its presence may be rendered conducive to the wants of the economy. In order to accomplish this, two kinds of changes may take place-1st, The exudation serves as a blastema, in which new vital structures originate and are developed; 2d, It exhibits no power of becoming organised, and the exuded matters, together with the textures involved in them, die. In the former case corpuscles spring up in the exudation,

which differ in form, size, constitution, and power of further development, and give rise to those various appearances and changes which in some cases have been denominated the result of inflammation, in others various kinds of deposits.

We find that the peculiar constitution of the blood, or the general vital power of the organism, exercises a very powerful influence on the development of the exudation. This has been long recognised by pathologists in certain conditions, denominated respectively diathesis, dyscrasia, or cachexia. I propose at present to direct your attention to some of the facts connected with exudation as it occurs in the body during health, as well as when connected with scrofulous and cancerous constitutions. I shall call the former simple exudation, as distinguished from what may be denominated tubercular and cancerous exudations.

1. Simple Exudation presents four principal forms-(a) As it occurs on serous membranes, when it exhibits a finely fibrous structure, and has a strong tendency to be developed into molecular fibres; (b) As it occurs on mucous membranes, or in areolar tissue, when it is generally converted into pus corpuscles; (c) When it occurs in dense parenchymatous organs, such as the brain, where it assumes a granular form, and is associated with numerous compound granular corpuscles; (d) As it is poured out after wounds or injuries, and occurs on granulating sores. In these cases the superficial portion is transformed into pus corpuscles, while that deeper seated is converted, by means of nuclei and cells, into nucleus and cell fibres, which ultimately form the cicatrix.

(a) On examining the minute structure of the exudation on a serous surface when recently formed, and when it presents a gelatinous semi-transparent appearance, it may be seen to be made up of minute filaments mingled with corpuscles. The filaments are not the result of the development of either a nucleus or a cell, but are formed by the simple precipitation of molecules, which arrange themselves in a linear manner, in the same way as they may be seen to form in the buffy coat of the blood. As the exudation assumes firmness, the filaments become more distinct and consolidated, and vary

from th tooth of a millimetre in diameter. Bundles, or different layers of them, often cross each other. As the lymph becomes older, they assume more and more the character of those in dense fibrous tissue. The corpuscles, when newly formed, are delicate and transparent, but in a short time become more distinct, and are then seen to be composed of a distinct cell wall, enclosing from three to eight granules. They vary in size from th to th, and the enclosed granules from th to th of a millimetre in diameter. On the addition of water and acetic acid, the corpuscles undergo no change, although sometimes the latter re-agent causes the cell wall to contract and thicken; and at others, to be somewhat more transparent. I have been in the habit of calling these bodies plastic corpuscles, from the frequency of their occurrence in plastic lymph. By Valentin and others, they have been named exudation corpuscles; and by Lebert, pyoid, from their resemblance to pus.

Molecular fibres and plastic corpuscles in lymph from a serous surface. a, the latter, after the addition of acetic

acid.

500

(b) Exudation poured out on a mucous membrane sometimes presents a fibrous mass, as in cases of croup and diphtheritis, but more generally it passes into an opaque, unctuous, straw-coloured fluid long known under the name of pus. When poured into the meshes of the areolar tissue, the same transformation occurs, constituting an abscess. On examining the minute structure of pus,

C

it is seen to be composed of numerous corpuscles floating in a clear fluid. These corpuscles are perfectly globular in form, and vary in size from the

Pus cells. a, fully formed; b, the same after the addition of acetic acid; c, liberated nuclei-ordinary

pus corpuscles; d, the same after

the addition of acetic acid.

Tooth to the th of a millimetre in diameter. Their surface is finely granular. They have a regular well-defined edge, and roll freely in the liquor puris upon each other. On the addition of water, they become much increased in size, their finely granular surface disappears, and they become more transparent. Weak acetic acid partially, and the strong acid completely, dissolves the cell-wall, and brings into view an included body, generally composed of two or three granules close together, and rarely of four or five, each with a central shadowed spot. These are usually about the 40th of a millimetre in diameter.

In some cases the pus corpuscles now described are surrounded by a distinct though delicate cellwall. The cell so formed is about the th of a millimetre in diameter, and is highly elastic, assuming different shapes, according to the degree and direction of the pressure to which it is subjected. Water and acetic acid cause the cell-wall to be at once dissolved, whilst the nucleus, which, before the addition of re-agents, exactly resembles an ordinary pus corpuscle, exhibits the usual body composed of two or three granules. What have hitherto, therefore, been called pus corpuscles, are the nuclei of cells, the delicate walls of which have been dissolved.

(c) In parenchymatous organs, the exudation insinuates itself among the

elementary tissues of which they are composed, so that when it coagulates, these are imprisoned in a solid plasma, like stones in the mortar of a rough cast wall. The whole then constitutes a firm mass, giving increased density to organs, a circumstance well observed in the lung, where, however, a mucous surface extensively prevails, and where the exudation is commonly transformed into pus. In the brain and spinal cord, we find it to be deposited in the form of minute molecules and granules, which are frequently seen coating the vessels externally, and filling up the intervascular spaces. The granules vary in size from theth to the th of a millimetre in diameter. They always contain among them round transparent globules, varying in size from the 30th tooth of a millimetre in diameter. These are the nuclei of round or oval cells which may frequently be observed in various stages of development. When fully formed, they vary greatly in size, for the most part measuring from the th to th of a millimetre in diameter. They sometimes contain a few granules only, at others they are so completely filled with them, that they assume a brownish-black appearance. Water and acetic acid cause no change in them, although the latter re-agent on some occasions renders the cell-walls more transparent. They are immediately soluble in ether, and break down into a molecular mass on the addition of potash and ammonia. These are compound granular cells. Masses of these granules may be occasionally seen floating about, of irregular shape, without any cell-wall. They are produced either by the solution of the cell-wall in which they were contained, or from the separation, or peeling off, of such masses from the external wall of the vessels. These are compound granular masses.