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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 [Gefässterritorien] (vascular unities).—Conveyance of nutriment in the juice-conveying canals (Saftkanäle) 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.

FIG. 28.

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 centralis or intralobularis, 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.

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the capillary network into the intralobular vein, whence, by means of the hepatic veins, it is conducted back again to the heart. Now, in the case of an injected liver, this network is seen to be so close that what interstices there are left seem almost to occupy less room than the vessels themselves. We can thus easily imagine how the older anatomists, such as Ruysch, came to be led by their injections to the supposition that nearly every thing in the body was made up of vessels, and that the different organs were only distinguished by differences in the arrangement of their vessels. But just the opposite to what is observed in an injected preparation does the proportion between vessel and tissue appear to be in an ordinary specimen from a liver. In this the vessels are scarcely perceptible. A similar network is indeed seen, but it is the network formed by the hepatic-cells (Fig. 27), which, closely crowded one against the other, fill up all the interspaces of the vessels. It is plain, therefore, that the capillary and hepatic-cell networks are interwoven in the most intricate manner, so that cells belonging to the parenchyma of the liver everywhere lie in almost immediate contact with the walls of the vessels, there being at most a fine layer between the cells and the walls, concerning which it is still a matter of dispute amongst histologists whether it is to be regarded as a peculiar coat, constituting the finest gall-ducts, or only as a very small quantity of connective tissue accompanying the vessels.

In this extremely simple case, a tolerably simple relation may certainly be assumed to exist between the vessels and the cells; it may be conceived that the blood which flows through the vessels may, in proportion to the degree in which they are contracted or dilated, and to its own quantity, exercise a direct influence upon the adjoining cells. It might indeed be objected, with regard to the conditions of nutrition, that we have here to deal with quite a pecu

VESSELS OF THE BRAIN.

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liar arrangement of the vessels, which are essentially of a venous nature, as being composed of ramifications of the portal and hepatic veins, but the hepatic artery also enters into the formation of this capillary network, so that the blood in it cannot be resolved into its individual arterial and venous constituents. Injections from each of the vessels named ultimately find their way into the same capillary network.

In most parts, however, the relations do not present such a simple form as in the liver; considerable interspaces often separate the individual cells, and no inconsiderable quantities of these elements are enclosed in every capillary mesh. I shew you here a second object derived from a fresh human brain-from a lunatic who died with his cerebrum in a highly hyperæmic state. The section

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has been made through the corpus striatum, which was of a deep red colour. You have a good view of the naturally

Fig. 29. Natural injection of the corpus striatum of a lunatic. a, a. Gaps destitute of vessels, and corresponding to the strands of nervous fibres which traverse the ganglion. 80 diameters.

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