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will not here treat in detail, but will only say as much as is necessary to give a cursory view of the matter.
A capillary vessel is a simple tube (Fig. 3, c), in which we have, with the aid of our present appliances, hitherto only been able to discover a simple membrane, beset at intervals with flattened nuclei, which, when seen in the middle of the surface of the vessel, present the same appearance as in the elements of muscle, only that they usually lie more at the sides, and therefore frequently have an awl-shaped appearance, from their sharp border alone being perceived. It is this, the most simple class of vessels, which we now a days solely and exclusively call capillaries, and with regard to them we cannot say that they become wider or narrower by means of any action of their own, but at most that their elasticity renders a certain degree of contraction possible. Nowhere are there to be witnessed in them genuine processes of contraction or relaxation succeeding it. The discussions which formerly took place with regard to the contractility of the capillaries really had reference to small arteries and veins, the calibre of which grows narrower through the contraction of their muscular coats, or wider upon the occurrence of relaxation in consequence of the pressure of the blood. This is one of the first facts, and an important one it is, which have resulted from the more accurate histological knowledge of the smaller and larger vessels, and it shows us that we cannot speak of the general properties of vessels, inasmuch as the capillaries differ essentially in structure from the small arteries and veins. These are composite structures, partaking of the nature of organs, whilst a capillary vessel is rather a simple histological element.
the cost is reduced to a single layer, with longitudinal nuclei, which here and there have been replaced by masses of fat-granules (fatty degeneration). b, b. Middle coat (circular fibrous, or muscular, coat), with long, cylindrical nuclei, which run transversely around the vessel, and at its borders (where they look as though they had been cut across) present the appearance of round bodies; at D and E transverse nuclei of the middle coat becoming continually scarcer. c, c. Internal coat, at D and E with longitudinal nuclei. 300 diameters.
Now that we have, gentlemen, completed a very general survey of the physiological tissues, the question arises, how the pathological ones in their turn comport themselves. By pathological tissues, of course, those only can be meant which really constitute pathological new formations, and not physiological parts which have simply undergone alteration in consequence of some deviation from the normal processes of nutrition. We have in them to deal with genuine neoplasms, with the additional matter furnished by the growth of new tissues in the course of pathological processes, and the question is, whether the general types which we have established for the physiological tissues will also be found to hold good in the case of the pathological ones. To this I unreservedly reply, yes; and however much I herein differ from many of my living contemporaries, however positively the peculiar (specific) nature of many pathological tissues has been insisted upon during the last few years, I will nevertheless endeavour in the course of these lectures to furnish you with proofs that every pathological structure has a physiological prototype, and that no form of morbid growth arises which cannot in its elements be traced back to some model which had previously maintained an independent existence in the economy.
The classification of pathological newformations, of genuine neoplasms, was formerly by most observers attempted to be based upon their different degrees of vascularity. If you examine the different treatises which appeared upon this subject up to the time of the cell-theory, you will find that the question of organization was always decided by that of vascularity. Every part which contained vessels was regarded as organized, and every part as unorganized which was destitute of vessels. But this, according to present notions, is an incorrect view of the matter, inasmuch as we have also physiological tissues without vessels, as for example, cartilage.
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But at a time when the more minute elements of tissues were at most only known as globules, and when very different virtues were attributed to these globules, it was quite excusable that everything should be referred to the vessels, particularly after the comparison John Hunter made between pathological new formations and the development of the chick in the egg, when he endeavoured to shew that, just as the punctum saliens in the hen's egg constitutes the first phenomenon of life, the vessels also were the first things to show themselves in pathological formations. You no doubt still remember how several "parasitical" new formations were described by Rust and Kluge as provided with an independent vascular system, which, without having any connection with the old vessels, developed itself quite independently, as is the case in the chick. Many attempts had indeed been made even before this to refer the apparently so irregular forms of new formations to physiological paradigms, and herein essential service has been rendered by natural philosophers. At the time when theromorphism played a conspicuous part, and many analogies were discovered between pathological processes and the normal states of inferior animals, comparisons also began to be instituted between new formations and familiar parts of the body. Thus, Johann Friedr. Meckel, the younger, spoke of mammary and pancreatic sarcoma. What has very recently been described in Paris as heteradenia (Heteradenie), or a heterologous formation of glandular substance, was in the school of the natural philosophers a pretty generally accepted fact.
Since the study of embryology has been prosecuted in a more histological manner, the conviction has gradually more and more been acquired, that most new formations contain parts which correspond to some physiological tissue, and in the micrographical schools of the west a certain number of observers have come to the conclusion, that in the whole series of new formations there is only one particular structure which is specifically different from natural formations, namely cancer. With regard to this, the most important points urged are, that it differs altogether from every other tissue, and that it contains elements sui generis, whilst, singularly enough, a second formation, between which and cancerous tissue the older writers were wont to draw parallels, namely tubercle, has—although to it too nothing strictly analogous could be discovered—been much neglected, owing to its having been regarded as an incomplete and somewhat crude product, and as a structure which had never become properly organized. Yet, upon a more careful examination of cancer or tubercle, we shall find that everything depends upon our searching for that stage in their development, in which they are exhibited in their perfect form. We must not examine at too early a period, when their development is incomplete, nor yet at too late an one, when it has proceeded beyond its highest point. If we restrict our observations to the time when development is really at its height, a physiological type may be found for every pathological formation, and it is just as possible to discover such types for the elements of cancer as to find them, for example, for pus, which, if it be sought to maintain the specific nature of certain formations, is just as much entitled to be regarded as something peculiar as cancer. Both of them stand upon precisely the same footing in this respect, and when the older writers spoke of cancer-pus they were in a certain measure right, inasmuch as cancer-juice is only distinguished from pus by the higher degree of development to which its individual elements have attained.
A classification of pathological structures also may be made upon exactly the same plan as that which we have already ventured upon in the case of the physiological tissues. In the first place, there are also among these structures some
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which, like the epithelial ones, are essentially composed of cellular elements, without the addition of anything else of consequence. In the second place, we meet with tissues which are allied to those called connective, inasmuch as in addition to the cells a certain quantity of intercellular substance is present. In the third and last place come those formations which are akin to the more highly organized structures, blood, muscles, nerves, &c. Now, a point to which I must at once direct your attention is, that in pathological formations those elements the more frequently exist, and the more decidedly prevail, which do not represent the higher grades of really animal development, and that, therefore, on the whole, those elements are most rarely imitated which belong to the more highly organized, and especially, to the muscular and nervous, systems. Still, these formations are by no means excluded; we find pathological new formations of every description, no matter to what tissue they may be analogous, provided it possess distinctive features. It is only with regard to their frequency and importance that a difference prevails, and this is of such a nature that the great majority of pathological productions contain cells analogous to epithelial cells, or to the corpuscles of the connective tissues, and that of those structures which we have included in the last class of normal tissues, the vessels and parts which may be compared with lymph and lymphatic glands are the most frequently met with as new formations, whilst real blood, muscles, and nerves, are the most seldom found as such.
But, if we ultimately arrive at such a simple view of the matter, the question of course arises, what becomes of the doctrine of the heterology of morbid products, to the upholding of which we have long been accustomed, and to which the most simple reflection almost inevitably conducts us. Hereunto I can return no other answer than that there is no other kind of heterology in morbid structures