According to the language of Abernethy, "a tumour consists in a swelling arising from some new production which made no part of the original composition of the body."

Now, in considering all the various tumours and indurations which occur in inflammation and disease, we have reason to suspect that the process by which they are formed must be attended with considerable diversity. Yet, notwithstanding this, the general principles of morbid changes of structure may admit of being reduced to a small number, for if we take the acknowledged products of inflammation, and to them add tubercle, scirrhus, fungus, and melanosis, we have at once a bird's-eye view of the most important changes which occur in the solids.

It appears, then, from the preceding observations, that a tumour is a new growth, or the deposit of an adventitious substance upon, amongst, or within the textures of the body, whereby these textures or organs acquire increased magnitude, so as to constitute themselves at once swellings, and to undergo changes or the obliteration of original structure. As an illustration, fungus hoematodes, or medullary cancer, may be cited, which is a new growth or deposit met with in three distinct forms, sometimes as a new product enclosed in a cyst, sometimes as a new formation unprovided with any cyst, and, in other instances, as an infiltration in the substance of an organ.

The process by which tumours are formed has sometimes been thought to be attended with an increased action of the vessels which supply them with blood. It has been compared to the process which forms all the thickenings and indurations met with under various circumstances in different parts of the body, and has often been referred to chronic inflammation. This subject of chronic or passive inflammation is one about which, so far as particular forms of it are concerned, very little certain is known, and even the name itself has commonly been admitted only on the supposition that some kind of increased action exists in the vessels, though of a slower and less evident kind than what prevails in acute inflammation.

Now, upon this part of the inquiry, different views are entertained. Three explanations have been offered of the mode in which tumours originate and grow. Firstly, the effusion of blood and its coagulation, and the subsequent organisation of the coagulum. Secondly, the effusion and organisation of coagulating lymph. ' Thirdly, chronic inflammation. Therefore, according to these explanations tumours ought to pass through successive stages, and to present different appearances at different periods of their development. For instance, we ought to find them at first as masses of coagulated blood, or coagulating lymph, and then to observe various degrees of transition from those substances to the textures which characterise the perfect growth. But this is not so, for a tumour in its earliest state and smallest size has its peculiar structure as well marked as in its subsequent progress and full development An adipose tumour, not exceeding the bulk of a pea, differs only in size from one as large as a goblet. Effusions of blood into the cellular texture from external violence are of daily occurrence; if, therefore, such extravasations could become organised, and then form tumours, they would prevail almost universally.

We see, however, that the blood thus poured out either disappears by absorption, or irritates the surrounding parts and causes suppuration, by which it is expelled.

It would appear, however, that coagula of blood and fibrine are capable of becoming organised under peculiar circumstances; because in the production of some kinds of tumours and new formations, little doubt, I think, can be entertained that the organisation of these substances, and especially coagulable lymph or fibrine, is an essential part of the process.

Dr. Carswell, who adopted the expression "analogous tissues" to signify all solid, morbid products which resemble the natural elementary tissues of the body, remarks that they present two important differences in regard to their origin. The plastic element of the blood, the spontaneously coagulable part of this fluid, or the fibrine, is by far the most frequent source, and furnishes the materials for the formation of the most perfect examples of the analogous tissue. It is to those which have this origin that the term analogous, accidental, adventitious, or pseudo-formations is correctly applied.

Another and entirely different origin of many of these tissues, is a change taking place in the primary or existing elementary tissues, and even in organs, by means of which they are converted into tissues of a different kind; as for example, when cartilage is converted into bone, or cellular into serous or fibrous tissue. The analogous tissues which have this mode of origin are, in order to distinguish them from the former, called analogous transformations. These tissues are again subdivided, and although originating in the plastic element of the blood, they may be formed out of this substance, whether it be separated from the blood, and effused on the surface of organs in the state of coagulable lymph in consequence of inflammation, or whether it be separated from this fluid, which had ceased to circulate in the vessels, or had escaped from them in consequence of mere physical causes. Now, in support of this doctrine, which maintains that analogous formations may originate in the fibrine of the blood, the evidence is derived from the changes which are observed to take place in the blood which has ceased to circulate in the heart or bloodvessels, or which has been effused into the substance of an organ. The cessation of the circulation, which precedes these changes in the blood, may be effected by the operation of mechanical, physical, and vital agents, which act either on the blood itself, or on the vessels in which it is contained. The most obvious examples of the operation of the first kind of agents are those in which the circulation is at once arrested, either in an artery or a vein, or both, by the application of a ligature, or in which the blood stagnates in the veins, for example, of the lower extremities, from a mechanical obstacle to its return situated in a remote part, or in the heart. The first change which the blood is observed to undergo in these circumstances is coagulation, the extent of which in an artery is almost always determined by the situation of the first branch of considerable size sent off from the obstructed vessel between the ligature and heart, but which in the veins varies with the situation of the obstacle, and the greater or less facilities afforded for the development of a collateral circulation. Whatever may be the extent of the coagulation, the subsequent changes which take place in the blood are as follow: The coagulum acquires gradually an increase of density, which is accompanied by the removal of the red colouring matter of the blood. The fibrine becomes thus more and more apparent, and is recognised by its pale straw colour, and more especially by the manifestation of its plastic properties, whereby it assumes, almost from the commencement, a laminated or fibriform arrangement. In this, the early stage of what may be called the process of organisation of the fibrine, there is one circumstance which is peculiarly interesting, not only because it enables us to explain the origin and mode of formation of some analogous tissues, but because it shows that the vital endowments, or plastic properties of fibrine under the circumstances in which we are now considering it, are of the same kind as those of coagul-able lymph, however much they may vary in degree.

The circumstance to which I allude, is the tendency of the fibrine, from the commencement, to escape towards, and to accumulate at, the circumference of the coagulum, or to place itself in contact with the living tissues which surround the coagulum. It is not only in a bloodvessel that this is observed; it is seen to take place in the cavity of the heart in the formation of what are called polypi, whatever may be their mode of attachment, as well when they occupy the greater part of one of the cavities of this organ, as when they are small, and connected only by a narrow pedicle.