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LAW OF CONTINUITY. 69

that the demonstration of the continuity of tissues must be regarded as a decisive proof of their intimate relationship. That as soon as one part could be made out to be continuously (by union, not mere juxtaposition) connected with another, both must be regarded as parts of a common whole. In this manner he sought to prove that cartilage, periosteum, bone, tendons, fasciae, &C., really formed a continuous mass, a kind of basis-tissue (Grundgewebe) for the body, a connective substance, which had only experienced certain changes in these different localities, without their being, however, of such a nature as to destroy the character of the tissue as such. This so-called law of continuity soon suffered the most violent shocks, and quite recently such a terrible breach has been made in it, that it can scarcely any longer be possible to derive therefrom any general criterion for the determination of the nature of a tissue. On the one hand, namely, new facts have been continually brought forward in support of the continuity of such histological elements as, according to Reichert, would be separated toto ccelo from one another, as, for example, of epithelial and connective tissue; and there has been a continually increasing mass of evidence in support of the assertion that cylindrical epithelium is capable of becoming elongated into fibres, which in the shape of filaments anastomose with connective-tissue corpuscles. Nay, it has been quite recently asserted by a whole series of observers that these superficial cells are prolonged inwardly, and then enter into direct connection with nerve-fibres. With regard to this last point, I must confess that I am not yet convinced of the correctness of the representation; but with respect to the former one, that is a matter which probably will end in the demonstration of the real continuity of the elements. It would seem, therefore, that it is even now no longer possible to mark out the exact limits which divide every kind of epithelium from every kind of connective tissue, but only where scaly epithelium is met with, whilst the limits are doubtful wherever cylindrical epithelium exists.

Just in the same manner elsewhere also do the boundary lines become obliterated. Whilst formerly the limits which separate the elements of muscle from those of tendon were considered to be most distinctly defined, extremely decisive proofs have in this case also been afforded, and first by Hyde Salter and Huxley, that fibres proceed from connective-tissue corpuscles, which, whilst pursuing their course in an inward direction, all at once assume the character of transversely striped muscle. So, then, in the case of connective tissue, it would seem there exists a continuous connection between the elements of the surface and the more highly developed ones of the deeper parts. Now if, on the other hand, it has turned out to be very probable that the corpuscles of connective tissue have definite relations to the vascular system, we are, as you see, almost justified in regarding this tissue as a kind of neutral ground for parts to meet upon (indifferenter Sammelpunkt), as a peculiar arrangement for their intimate connection, an arrangement which, though certainly not exercising any great influence upon the higher functions of the animal, is yet of great importance as far as its nutrition is concerned.

In the place of the law of continuity, therefore, we must necessarily put something else. And here, I think, the doctrine which has the strongest claims to our attention is that of histological substitution. In the case of all tissues of a like nature it is quite possible, even whilst confining our attention to what occurs physiologically in the various classes of animals, to find one tissue at a certain fixed point of the body replaced by an analogous one belonging to the same group, or, in other words, by an histological equivalent.

A spot invested with cylindrical, may acquire scaly, epithelium. A surface upon which cilia were originally seen, HISTOLOGICAL EQUIVALENTS AND SUBSTITUTIONS. 71

may afterwards be found to have ordinary epithelium. Thus, on the surface of the ventricles of the brain we meet at first with ciliated, and at a later period with simple scaly, epithelium. Thus, too, we see the mucous membrane of the uterus usually covered with ciliated epithelium, but during pregnancy we find the layer of ciliated cylinders replaced by one of squamous epithelium. Thus, also, in places where soft epithelium ordinarily is found, epidermis may, under particular circumstances,be generated, as, for example, in the prolapsed vagina. Thus, again, in the sclerotic coat of the eyes of fish, cartilage is found, whilst in man this tunic consists of dense connective tissue; in many animals bone is found in parts of the skin, where in man there is only connective tissue; but in man, too, in many places where there was originally cartilage, osseous tissue is afterwards discovered. But the most striking instances of such substitutions are met with in muscles. One animal has transversely striped muscular fibres in the same place that another has smooth ones.

In diseased conditions pathological substitutions occur, in which a given tissue is replaced by another; but even when this new tissue is produced from the previously existing one, the new formation may deviate more or less from the original type. There is therefore a great chasm between physiological and pathological substitution, or at least between the physiological and certain forms of the pathological one.

Physiologically, the substitution is constantly effected by the introduction of another tissue of the same group (homology) ; pathologically, very frequently by the agency of a tissue belonging to another {heterology). To this we must reduce the whole doctrine of the specific elements of pathology which have played so conspicuous a part in the last twenty years.

LECTURE IV.

FEBRUARY 24, 1858.
NUTRITION AND ITS CHANNELS.

Action of the vessels-- Relations between vessels and tissues.—Liver.—Brain.— Muscular coat of the stomach.—Cartilage.—Bone.

Dependence of tissues upon vessels.—Metastases.—Vascular territories [Gefassterritorien] (vascular unities).—Conveyance of nutriment in the juice-conveying canals (Saftkanale) of the tissues.—Bone.—Teeth.—Fibro-cartilage.—Cornea.—Semilunar cartilages.

According to the ideas usually entertained with regard to nutrition, the vessels are regarded as the canals by means of which not only the interchange of material (Stoffverkehr) is accomplished, but upon the assistance of which, sometimes actively and sometimes passively afforded, reliance is placed whenever it is required to control an individual part in its interchange of material. The regulating principle in the process of nutrition was long designated by an expression which has even crept into the language of the present day, namely, the "action of the vessels," as if they were endowed with a special power of actively influencing the condition of the neighbouring histological constituents.

As I pointed out to you the last time, when upon the subject of muscular fibres (p. 57), we can now a days only speak of action in the vessels in as far as muscular fibres are present in them, and the vessels are thus enabled by the contraction of these fibres to grow narrower or

CAPILLARY VESSELS OF THE LIVER. 73

shorter. This narrowing of their channel may have the effect of impeding the transudation of fluids, whilst, on the contrary, in the case of the relaxation or paralysis of the muscular fibres, the widening of the vessel may favour such transudation. Let us admit this for the present, but allow me, before proceeding farther, to enter somewhat into the analysis of the mass of tissue which lies around the vessels, and is generally conceived to be of a very simple and uncomplicated nature.

If we select parts where the vessels lie very closely packed, and there is perhaps nearly as much vessel as tissue, as, for example, the liver, in which this condition really does occur (for a liver, when its vessels are full, contains nearly as large a volume of vessels as it does of proper hepatic substance), we see that the interstices which are left between the vessels are filled with quite a small number of cells.

If we examine a single acinus of the liver by itself, we find, when a very lucky transverse section has been obtained, in its centre the vena „ ao ,. -iii- *IG-2°centralis or intralobular^,

which runs into the hepatic vein, and at the periphery branches of the portal vein, which send capillary twigs into the interior. These at once form a network, which at first has long, but afterwards more regularly shaped, meshes, and extends in the direction of the central (or hepatic) vein, and at last terminates in it. The blood, therefore, after it has entered by the interlobular (or portal) vein, flows through

Fig. 28. Section from the periphery of the liver of a rabbit; the vessels completely injected. 11 diameters.

[graphic]
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