place in the part from which the vessels emptying into these trunks proceed, effusion of blood, or of serous fluid results from the distended state of the capillaries, the functions of the part are impaired or wholly deranged, and, if the obstruction be complete or nearly so, mortification occurs as a necessary consequence of the want of those properties in the retained blood which are essential to the support of all the vital actions. If, on the contrary, the artery be obstructed, the part in which it ramifies languishes from the want of blood, and may perish from the same cause, if the caliber of the vessels be quite obliterated, and the blood be unable to find its way through the anastomosing branches. Obstruction of the absorbents gives rise to tumefaction, partly from the distended state of the vessels themselves behind the part affected, partly from the accumulation of effused fluid consequent upon deficient absorption. The obstructing cause is often such as to affect more than one of these sets of vessels, and the results are modified accordingly.

The causes of obstruction may be external, as ligatures around the limbs or neck, tight lacing, and pressure either by the weight of the body, or from some extraneous source; or they may be internal, as tumours, enlarged or indurated glands, aneurisms, the impregnated uterus, diseased contractions or thickening of the coats of the vessels, or the presence of coagulated blood or fibrin within their caliber.

Obstruction to the course of the blood, and consequent congestions, may arise also from a loss of due proportion between the cavities of the heart, and from contraction or insufficiency of the valves; but this subject will be fully considered under the head of diseases of the heart.

Any impediment to the capillary circulation in organs supplied by the ramification of large vessels conveying venous blood, must prove an obstruction to the circulation in these vessels, and consequently a cause of congestion in those parts of the body from which they proceed. Thus, if the capillaries of the liver, whether from constriction produced by irritation, from debility, or from any other cause, cease to convey the blood through this organ with the usual rapidity, the portal vein becomes necessarily distended, and congestion takes place in the stomach, bowels, and all those parts the veins of which empty into that great trunk. So also, in the case of a similar impediment suddenly produced in the capillary circulation of the lungs, as in the instance of death from asphyxia, the pulmonary arteries, the right cavities of the heart, and consequently the general venous system become congested, as is evinced by the bloated and livid countenance, the prominent eyes, and the whole surface injected with dark blood, giving the skin a mottled appearance. 5. Obstructions in the excretory ducts of glands, or other passages intended for the escape of secreted fluids, give rise to a series of morbid phenomena, which, in their first step at least, must be considered in the light of mechanical affections. The fluid of which the outlet is closed accumulates in the tubes or receptacles behind the point of obstruction, producing distension, which is in some instances enormous, and which, unless relieved, results in a rupture of the coats, and not unfrequently fatal effusion of the liquid into neighbouring parts. Another result is either a re-entrance into the circulation, by means of absorption, of the secreted fluid or some of its constituents, or the retention in the circulation of principles which would otherwise be thrown off by secretion; this process being arrested by the pressure of the accumulated fluid. As some of these principles are noxious when existing in excess in the blood, very serious effects are occasionally produced. Thus, when the duct which conveys the bile into the intestines is closed, this liquid necessarily accumulates behind the point of closure, and there is danger of rupture of the ducts or of the gall bladder, and of

fatal effusion into the abdominal cavity. The colouring matter and probably other principles of the bile may be absorbed; and, as the secretory action of the liver is restrained by the pressure of the fluid accumulated in the ducts, the bilious matter ceases to be eliminated as rapidly as usual, and consequently, from this cause also, becomes redundant in the blood. Here it acts as a poison, producing various morbid sensations, which are partially relieved by the escape of the offending cause by different emunctories, as by the skin and kidneys, giving rise to jaundice. A similar train of consequences follows an obstruction of the ureters.

The causes of such obstruction may be external to the tubes, as indurated tumours in neighbouring parts; or they may consist in an altered state of the coats of the tubes, as thickening or spasmodic contraction; or, finally, they may be within the cavity of the tubes, as calculous concretions, or viscid and impacted secretions.

6. Lastly, under the head of mechanical affections, may be enumerated some of the immediate consequences of obstruction in the respiratory passages and alimentary canal; as suffocation from a foreign body either in the larynx, or in the oesophagus pressing upon the trachea; inability to swallow from stricture of the oesophagus, from a foreign body in that tube, or from pressure made upon it by a tumour; and stoppage of the contents of the stomach and bowels from stricture, intussusceptio, strangulated hernia, or accumulated and hardened feces. But in these and similar cases, the me chanical results are so intermingled with vital derangements, or so immediately followed by them, and the latter are so much the more important, that the cases are most advantageously treated of in connexion with these derangements.

SUBSECTION II.

DISEASE FROM INFLUENCES UPON THE VITAL PROPERTIES.

All living parts are endowed with the property of excitability, or in other words are capable of being brought into and maintained in action by the influence of certain agents either internal or external. The excitation thus produced is essential to the preservation of life, and is susceptible of considerable variation in degree or extent, without exceeding or falling short of the limits of health.

But, under the influence of certain causes, this excitation may be increased or diminished so as to occasion uneasiness to the individual, and interfere with the due performance of the functions. Even ander these circumstances, it can scarcely be considered in the light of disease, if the variation from the due standard be of very brief duration. Thus heat and cold, in the ordinary changes of the seasons, often produce painful impressions, the former with an augmented, the latter with a diminished action of the part affected; but, if these impressions disappear immediately on the removal of the cause, leaving no permanent derangement, they do not constitute disease. It is, indeed, impossible to say where precisely, in the advance or retrocession of excitation, health ends and disease commences.

The morbid excitation may be such as to affect only the function of the part or organ which is the seat of it, or it may be attended with a change of structure. Some pathologists have, indeed, denied the existence, in any case, of functional without organic change. But, certainly, it is not impossible that a part may act in one way or another; that the heart, for example, may beat more or less frequently; that more or less liquid may separate from the blood; that a more or less rapid nutrition may take place, without alteration of structure. Whether, therefore, in all such cases, there is or is

not organic change, must rest upon observation; and we every day witness instances of disease in which no such change is discoverable; as in the various neuralgic affections. As we cannot draw the line precisely between health and disease, neither can we determine always where functional disorder ends and organic begins. The latter is usually preceded by the former; and, as a general rule, functional disorder long continued, ends in structural lesion.*

But all disorder is by no means the consequence of augmented action. A depression of action below the standard of health is equally a morbid state. This may arise directly from the application of sedative agents, or the withdrawal of the ordinary healthy excitants, or indirectly in consequence of certain physiological laws, to be explained in another place. It may be only functional, or may be attended with organic change, and result even in the death of the part affected.

In the case both of morbid exaltation and morbid reduction of the vital actions, the excitation and depression may be in the line of health, though above or below it; or they may deviate from this line, and present peculiar characters independent altogether of mere difference in degree. This has been denied by certain pathologists, who maintain that all disease is a mere augmentation or diminution of the actions of health, and that one disease differs from another only in degree or locality. But the instances are so numerous in which affections, occupying the same structure, present characters entirely different, independently, to all appearance, of the grade of action, or even of peculiarities in the organization of the part affected, that the position above stated must be held true, unless the phenomena can be demonstrated to be other than they seem. To adduce only a single example; a gland in the groin may become the seat of different diseases; one of which shall result in the production of healthy pus, followed by speedy restoration to health, as common inflammation; a second, in the generation of a peculiar virus, capable of producing the same disease in another, as syphilis; a third, in the deposition of a cheesy matter, with slow suppuration and very lingering recovery, as scrofula; a fourth, in characteristic degeneration of structure, utterly different from anything seen in health, or in other forms of disease, as cancer. No satisfactory explanation has yet been given of these and numerous other diversities, upon the principle of the unity of morbid action; and the beauty or simplicity of a theory is not a sufficient reason for admitting its validity, in opposition to the evidences of the senses.

There are, moreover, diseased conditions, which are not in themselves necessarily connected with an elevation or depression of the vital actions, but exhibit a complete change or perversion of them, and the substitution of others entirely different, as if belonging to a new order of parts.

On each of the morbid states alluded to in the preceding remarks, I shall offer some general observations, in order to avoid the repetitions which might

The author does not deny that every act of every part of the living body is accompanied, as essential to its performance, with some molecular change, though this is not positively proven, and is perhaps insusceptible of proof; yet such change may certainly take place without any alteration of the organization. If one molecule, or granule, or cell disappears, and is replaced by another exactly similar, the state of the organ remains the same; there has been performance of function without change of structure. Now it is easy to conceive that this alternate destruction and repair may go on more rapidly than is consistent with a healthy state of the function; the structure of the part remaining unaffected, and the same at the end as at the beginning of the morbid process. For the sake of convenience, we may consider organic derangement as commencing when the changes of structure are such as to be discoverable by the senses, and whatever falls short of this to be functional. (Note to the third edition.)

otherwise be necessary in describing the various diseases of which they are the constituents. It is essential that they should receive specific names, so as to spare continual circumlocution. I shall employ in general well known and accepted terms. Should the meaning here attached to them differ in any respect from their ordinary acceptation, I shall at least endeavour to make their use in this work correspond with the explanation given in the outset. That condition of super-excitation in which the functions are morbidly deranged, without obvious organic change, is called irritation. When the excitation is marked by peculiar characters, distinguishing it from the ordinary results of over-stimulation, it is designated as a specific irritation. The term irritation is also employed, even when organic changes take place, provided they fall short of that peculiar condition, usually marked by redness, heat, pain, and tumefaction, and which is denominated inflammation. When this condition occurs, the irritation is considered as merged in it, and the term is abandoned. Inflammation may also be marked, like the lower grade of excitation, by peculiar characters, materially differing from those which it presents in its ordinary form. Such deviations are distinguished by the name of specific inflammations. It is impossible in all cases to designate the precise boundary between irritation and inflammation. The lower grades of the former and the higher grades of the latter are very clearly marked; but they run together in the interval. They are merely opposite extremes of the same line of excitation.

That morbid condition in which the vital actions are reduced below the healthy standard is called depression. It differs from debility, which implies a loss or diminution of vital power; while, in the case of depression, this power may be undiminished, though temporarily restrained. It may consist in a simple reduction of the vital actions, or may be accompanied by peculiarities arising from the operation of peculiar causes, in which case it may be denominated specific depression. It may, moreover, be purely functional, or attended with organic changes. In the latter case, its lowest grade is mortification or gangrene.

Diseased conditions which are entirely peculiar are of course designated by special names, which it will be sufficient to mention when the diseases themselves are considered.

I shall proceed now to treat of the several morbid states above indicated; viz., irritation, inflammation, and depression, with their general causes, laws, and effects. As one of the attendants upon these morbid states, and deserving of notice from its frequent occurrence, as well as for the great importance attached to it by modern pathologists, I shall next consider the subject of congestion. The next general topic will be fever, which, though not elementary, is a most important constituent of numerous diseases, and, as it is perhaps incapable of analysis, may be advantageously considered among the simpler forms. Lastly, I shall treat, in a general way, of those peculiar morbid states, in which there is a complete change of the vital actions, and the production of new deposits or structures, giving rise to a series of phenomena peculiar to each of the states in question. Such are the tuberculous disease, melanosis, and cancer. Under these several heads, may be included all the constituent forms of disease, with the exception of certain morbid changes in the state of the system independent of deranged action, such as excess of the sthenic condition, debility, and excess or deficiency of excitability, which, however, may be incidentally considered in connexion with their attendant disorder of function.

Article I.

IRRITATION.

Causes, Propagation, Transfer, &c., of Irritation.-Any morbid excitement of the vital actions not amounting to inflammation is denominated irritation. This may result directly from an excess of the ordinary agents necessary for the support of health, such as food, drink, air, heat, light, and electricity; from the action of substances having acrid or stimulant properties, whether extraneous to the body, as various medicines and poisons, or produced within it, as acrid secretions, healthy secretions too long retained, products of chemical changes, parasitic animals, even the blood itself if too abundant or of altered qualities; from excessive or perverted mental action; and from various unknown causes, the existence of which is inferred from their apparent effects, as miasmata, contagion, and epidemic influence.

Irritation may also result indirectly, through certain physiological laws, which determine this condition as the necessary consequence of other antecedent conditions.

The system appears to possess a certain amount of excitability. If this be called into excessive action, it is exhausted in proportion to the excess, and there is necessity for subsequent repose in order that it may be recruited. If, on the contrary, from a diminution of the wonted stimuli, or by the action of directly depressing agents, it is allowed to rest, or is restrained, it is afterwards, upon the removal of the cause, proportionably augmented; and the amount of excitant agency, necessary under ordinary circumstances for the support of health, becomes now the cause of excessive and perhaps morbid action. Hence, depressing causes, if of temporary duration, indirectly produce irritation by the reaction which results. This is probably true of the whole system, though the fact is denied by some pathologists. It is certainly true of the several parts of the system. A familiar illustration is offered in the irritation which frequently takes place in the hand, previously exposed to severe cold, upon entering a warm apartment. A degree of heat which was before only sufficient to sustain the natural condition of the part, now that the excitability has accumulated through the sedative agency of the cold, becomes capable of producing an unhealthy excess of action. This principle is very important, and of very frequent applicability in the explanation of morbid phenomena.

There is another mode in which irritation may result indirectly from depressing causes. In the mode just described, the irritation occupies the precise seat of the antecedent depression, and succeeds it; in that now alluded to, it occurs in other organs, and is co-existent with the depression. When the healthy actions of a portion of the frame are reduced, the nervous energy and circulating fluid, being diminished in that part, necessarily accumulate elsewhere; and, if the cause continue to operate, a morbid excess of action, in other words, an irritation in some other part or organ is the consequence. This results in part from a mere change in the balance of the circulation, throwing an undue portion of blood, which is itself a powerfully stimulating agent, upon a particular organ; but probably in part also from the operation of that physiological law by which the system, possessing a certain amount of excitability, experiences a temporary increase of this property in certain parts, when its exercise is restrained in others. Thus we have, at the same