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[216] The Lecturer commenced by referring to his endeavours last year to show that the distinction between living creatures and those which do not live, consists in the fact, that while the latter tend to remain as they are, unless the operation of some external cause effect a change in their condition, the former have no such inertia, but pass spontaneously through a definite succession of states–different in kind and order of succession for different species, but always identical in the members of the same species.
There is however another character of living bodies–Organization; which is usually supposed to be their most striking peculiarity, as contrasted with beings which do not live; and it was to the essential nature of organization that the Lecturer on the present occasion desired to direct attention.
An organized body does not necessarily possess organs in the physiological sense–parts, that is, which discharge some function necessary to the maintenance of the whole. Neither the germ nor the lowest animals and plants possess organs in this sense, and yet they are organized.
It is not mere external form, again, which constitutes organization. On the table there was a lead-tree (as it is called) which, a mere product of crystallization, possessed the complicated and graceful form of a delicate Fern. If a section were made of one of the leaflets of this tree, it would be found to possess a structure optically and chemically homogeneous throughout.
Make a section of any young portion of a true plant, and the [217] result will be very different. It will be found to be neither chemically nor optically homogeneous, but to be composed of small definite masses containing a large quantity of nitrogen, imbedded in a homogeneous matrix having a very different chemical composition; containing in fact abundance of a peculiar substance–Cellulose.
The nitrogenous bodies may be more or less solid or vesicular–and they may or may not be distinguished into a central mass (nucleus of Authors) and a peripheral portion (Contents, Primordial utrtcle of Authors)–on account of the confusion in the existing nomenclature, the Lecturer proposed the term Endoplasts for them.
The cellulose matrix, though at first unquestionably a homogeneous continuous substance, readily breaks up into definite portions surrounding each Endoplast;–and these portions have therefore conveniently, though, as the Lecturer considered, erroneously, been considered to be independent entities under the name of Cells:–these, by their union, and by the excretion of a hypothetical intercellular substance, being supposed to build up the matrix. On the other hand, the Lecturer endeavoured to show that the existence of separate cells is purely imaginary, and that the possibility of breaking up the tissue of a plant into such bodies, depends simply upon the mode in which certain chemical and physical differences have arisen in the primarily homogeneous matrix, to which, in contradistinction to the Endoplast, he proposed to give the name of periplast or periplastic substance.
In all young animal tissues the structure is essentially the same, consisting of a homogeneous periplastic substance with imbedded Endoplasts (nuclei of Authors); as the Lecturer illustrated by reference to diagrams of young Cartilage, Connective Tissue, Muscle, Epithelium, &c., &c.; and he therefore drew the conclusion that the common structural character of living bodies, as opposed to those which do not live, is the existence in the former of a local physico-chemical differentiation; while the latter are physically and chemically homogeneous throughout.
These facts, in their general outlines, have been well known since the promulgation, in 1838, of the celebrated Cell-theory of Schwann. Admitting to the fullest extent the service which this theory had done in Anatomy and Physiology, the Lecturer endeavoured to show that it was nevertheless infected by a fundamental error, which had introduced confusion into all later attempts to compare the vegetable with the animal tissues. This error arose from the circumstance that, when Schwann wrote, the primordial utricle in the vegetable-cell was unknown. Schwann, therefore, who started in his comparison of [218] Animal with Vegetable Tissues from the structure of Cartilage, supposed that the corpuscle of the cartilage cavity was homologous with the "nucleus" of the vegetable-cell, and that therefore all bodies in animal tissues, homologous with the cartilage corpuscles, were "nuclei." The latter conclusion is a necessary result of the premises, and therefore the Lecturer stated that he had carefully re-examined the structure of Cartilage, in order to determine which of its elements corresponded with the primordial utricle of the plant,–the important missing structure of which Schwann had given no account:–-working subsequently from Cartilage to the different tissues, with which it may be traced into direct or indirect continuity, and thus ascertaining the same point for them.
The general result of these investigations may be thus expressed: –In all the animal tissues the so-called nucleus (Endoplast) is the homologue of the primordial utricle (with nucleus and contents) (Endoplast) of the Plant, the other histological element being invariably modifications ofthe periplastic substance.
Upon this view we find that all the discrepancies which had appeared to exist between the Animal and Vegetable Structures disappear, and it becomes easy to trace the absolute identity of plan in the two,–the differences between them being produced merely by the nature and form of the deposits in, or modifications of, the periplastic substance.
Thus in the Plant, the Endoplast of the young tissue becomes a "primordial utricle," in which a central mass, the "nucleus," may or may not arise; persisting for a longer or for a shorter time, it may grow, divide, and subdivide, but it never (?) becomes metamorphosed into any kind of tissue.
The periplastic substance follows to some extent the changes of the endoplast, inasmuch as it generally, though not always, grows in when the latter has divided, so as to separate the two newly formed portions from one another; but it must be carefully borne in mind, though it is a point which has been greatly overlooked, that it undergoes its own peculiar metamorphoses quite independently of the endoplast.–This the Lecturer illustrated by the striking case of the Sphagnum leaf, in which the peculiarly thickened cells can be shown to acquire their thickening fibre after the total disappearance of the primordial utricle,–and he further quoted M. von Mohl's observations as to the early disappearance of the primordial utricle in woody cells in general.–in confirmation of the same views.
Now, these metamorphoses of the periplastic substance are twofold: 1, Chemical; 2, Morphological.
[219] The Chemical changes rnay consist in the conversion of the cellulose into xylogen, &c., &c., or in the deposit of salts, silica, &c., in the periplastic substance. Again, the periplastic substance around each endoplast may remain of one chemical composition, or it may be different in the outer part (so-called intercellular substance) from what it is in the inner (so-called cell-wall).
As to Morphological changes in the periplastic substance, they consist either in the development of cavities in its substance–vacuolation (development of so-called intercellular passages) or in fibrillation (spiral fibres, &c.).
It is precisely the same in the Animal.
The Endoplast may here become differentiated into a nucleus and a primordial utricle (as sometimes in Cartilage) or more usually it does not,–one or two small solid particles merely arising or existing from the first, as the so-called "nucleoli,"–it persists for a longer or shorter time; it divides and subdivides, but it never (except perhaps in the case of the spermatozoa and the thread-cells of Medusæ, &c.) becomes metamorphosed into any tissue.
The periplastic substance, on the other hand, undergoes quite independent modifications. By chemical change or deposit it acquires Horn, Collagen, Chondrin, Syntonin, Fats, Calcareous Salts, according as it becomes Epithelium, Connective Tissue, Cartilage, Muscle, Nerve or Bone, and in some cases the chemical change in the immediate neighbourhood of the endoplast is different from that which has taken place exteriorly,–so that the one portion becomes separable from the other by chemical or mechanical means;–whence, for instance, has arisen the assumption of distinct walls for the bone-lacunæ and cartilage cavities; of cell-contents and of intercellular substance as distinct histological elements.
The Morphological changes in the periplastic substance of the animal, again, are of the same nature as in the plant:–Vacuolation and Fibrillation (by which latter term is understood, not only the actual breaking up of a tissue in definite lines, but the tendency to do so)–Vaculolation of the periplastic substance is seen to its greatest extent in the "Areolar" connective tissue;–Fibrillatton in tendons, ribro-cartilages and muscles.
In both Plants and Animals, then, there is one histological element, the Endoplast, which does nothing but grow and vegetatively repeat itself; the other element, the periplastic substance, being the subject of all the chemical and morphological metamorphoses, in consequence of which specific Tissues arise. The differences between the two kingdoms are, mainly: 1. That in the Plant the Endoplast [220] grows, and, as the primordial utricle, attains a large comparative size;–while in the Animal the Endoplast remains small, the principal bulk of its tissues being formed by the periplastic substance; and, 2; in the nature of the chemical changes which take place in the periplastic substance in each case. This distinction.however does not always hold good, the Ascidians furnishing examples of animals whose periplastic substance contains cellulose.
"The Plant, then, is an Animal confined in a wooden case, and Nature, like Sycorax, holds thousands of 'delicate Ariels' imprisoned within every Oak. She is jealous of letting us know this, and among the higher and more conspicuous forms of Plants, reveals it only by such obscure manifestations as the shrinking of the Sensitive Plant, the sudden clasp of the Dionœa, or, still more slightly, by the phenomena of the Cyclosis. But among the immense variety of creatures which belong to the invisible world, she allows more liberty to her Dryads; and the Protococci, the Volvox, and indeed all the Algæ, are, during one period of their existence, as active as animals of a like grade in the scale. True they are doomed eventually to shut themselves up within their wooden cages and remain quiescent, but in this respect they are no worse off than the Polype, or the Oyster even."
In conclusion, the Lecturer stated his opinion that the Cell-theory of Schwann consists of two portions of very unequal value, the one anatomical, the other physiological. So far as it was based upon an ultimate analysis of living beings and was an exhaustive expression of their anatomy, so far will it take its place among the great advances in Science. But its value is purely anatomical, and the attempts which have been made by its author, and by others, to base upon it some explanation of the Physiological phenomena of living beings by the assumption of Cell-force, Metabolic-force, &c., &c., cannot be said to be much more philosophical than the old notions of "the actions of the vessels," of which physiologists have lately taken so much pains to rid themselves.
"The living body has often, and justly, been called, 'the House we live in ;'–suppose that one, ignorant of the mode in which a house is built, were to pull it to pieces, and find it to be composed of bricks and mortar,–would it be very philosophical on his part to suppose that the house was built by brick-force? But this is just what has been done with the human body.–We have broken it up into 'cells,' and now we account for its genesis by cell-force."
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