Bridgewater Treatise V: Roget, Animal and Vegetable Physiology

This electronic edition prepared by Dr. David C. Bossard
from original documents in his personal library.

[009] THE mechanical structure and properties of the organized fabric, which have occupied our attention in the preceding volume, are necessary for the maintenance of life, and the exercise of the vital powers. But, however artificially that fabric may have been constructed, and however admirable the skill and the foresight which have been displayed in ensuring the safety of its elaborate mechanism, and in preserving the harmony of its complicated movements, it yet of necessity contains within itself the elements of its own dissolution. The nimal machine, in common with every other mechanical contrivance, is subject to wear and deteriorate' by constant use. Not only in the greater movements of the limbs, but also in the more delicate actions of the internal organs, we may trace the operation of many causes inevitably leading to their ultimate destruction. ... Provision must accordingly be made for remedying these constant causes of decay by the supply of those peculiar, materials which the organs require for recruiting their declining energies. ...[012] We may take as an example one of the simplest of organic products, namely, Sugar; a substance which has been analyzed with the greatest accuracy by modern chemists: yet to reproduce this sugar, by the artificial combination of its simple elements, is a problem which has hitherto baffled all the efforts of philosophy. [Note: The complex sucrose cycle was first described in the 1960s, 130 years after this writing -- dcb]

[012] The elements of organic substances are not very numerous: the principal of them being oxygen, carbon, hydrogen, nitrogen, sulphur, and phosphorus, together with a few of the-alkaline, earthy, and metallic bases. These substances are variously united, so as to form certain specific compounds, which, although they are susceptible, in difibrent instances, of endless modifications, yet possess such a general character of uniformity, as to allow of their being arranged in certain classes; the most characteristic substance in each class constituting what is called a proximate organic principle. Thus, in the vegetable kingdom we have Lignin, Tannin, Mucilage, Oil, Sugar, Fecula, &c. The animal kingdom, in like manner, furnishes Gelatin, Albumen, Fibrin, Mucus, Entomoline, Elearin, Stearin, and many others.

[013] This peculiar mode of combination gives rise to a remarkable condition, which attaches to the chemical properties of organic compounds. The attractive forces, by which their several ingredients are held together, being very numerous, require to be much more nicely balanced, in order to retaim them in combination. Slight causes are sufficient to disturb, or even overset, this equipoise of affinities, and often produce rapid changes of form, or even complete decomposition. The principles, thus retained in a kind of forced union, have a constant tendency to react upon one another and to produce, from slight variations of circumstances, a totally new order of combinations. Thus, a degree of heat which would occasion no change in most mineral substances. will at once effect the complete disunion of the elements o an animal or vegetable body. Organic substances are, in like manner, unable to resist the slower, but equally destructive agency of water and atmospheric air; and they are also liable to various spontaneous changes, such as those constituting fermentation and putrefaction, which occur when their vitality is extinct, and when they are consequently abandoned to the uncontrolled operation of their natural chemical affinities. This tendency to decomposition may, indeed, be regarded as inherent in all organized substances, and as requiring for its counteraction, in the living system, that perpetual renovation of materials which is supplied by the powers of nutrition. [emphasis added -- dcb] ... [014] Hence, the continued interchange and renewal of particles which take place in the more active organs of the system, especially in the higher classes of animals. In the fabric of those animals which possess an extensive system of circulating and absorbing vessels, the changes which are effected are so considerable and so rapid, that even in the densest textures, such as the bones, scarcely any portion of the substance which originally composed them is permanently retained in their structure. To so great an extent is this renovation of materials carried on in the human system, that doubts may very reasonably be entertained as to the identity of any portion of the body after the lapse of a certain time. The period assigned by the ancients for this entire change of the substance of the body, was seven or eight years: but modern inquiries, which show us the rapid reparation that takes place in injured parts, and the quick renewal o1 the bones themselves, tend to prove that even a shorter time than this is adequate to the complete renovation of every portion of the living fabric.

[015] Imperfect as is our knowlcdgc of organic chemistry, we see enough to convince us that a series of the most refined and artificial operations is required, in order to bring about the complicated and elaborate arrangements of elements which constitute both animal and vegetable products. Thus, in the very outset of this, as of every other inquiry in Physiology, we meet with evidences of profound intention and consummate art, infinitely surpassing not only the power and resources, but even the imagination of man [emphasis added].

1. Food of Plants. 19 019 020 021
2. Absorption of Nutriment by Plants. 21 021 022 023 024 025 026 027
3. Exhalation. 27 027
4. Aeration of the Sap. 28 028 029 030 031 032

[028] The remarkable discovery that oxygen gas is exhaled from the leaves of plants during the day time, was made by the great founder of pneumatic chemistry, Dr. Priestly. ... Solar light is an essential agent in effecting this....

[030] During the whole of the night, the same leaves, which had been exhaling oxygen during the day, absorb a portion of that element. The oxygen thus absorbed enters immediately into combination with the carbonaceous matter in the plant, forming with it carbonic acid. ...

This reversal at night of what was done in the day may, at first sight, appear to be at variance with the unity of plan, which we should expect to find preserved in the vegetable economy; but a more attentive examination of the process will show that the whole is in perfect harmony, and that these contrary processes are both of them necessary, in order to produce the result intended.

The water which is absorbed by the roots generally carries with it a certain quantity of soluble animal or vegetable materials, which contain carbon. This carbon is transmitted to the leaves, where, during the night, it is made to combine, with the oxygen they have absorbed. It is thus converted into carbonic dcid, which, when daylight prevails, is decomposed; the oxygen being dissipated, and the carbon retained. It is evident that the object of the whole process is to obtain carbon in that precise state of disintegration, to which it is reduced at the moment of its separation from carbonic acid by the action of solar light on the green substance of the leaves; for it is in this state alone that it is available in promoting the nourishment of the plant, and not in the crude condition in which it exists when it is pumped up from the earth...

[032] Thus are the two great organized kingdoms of the creation made to co-operatein the execution of the same design; each ministering to the other, and preserving that due balance in the constitution of the atmosphere, which adapts it to the welfare and activity of every order of beings, and which would soon be destroyed, were the operations of any one of them to be suspended. It is impossible to contemplate so special an adjustment of opposite effects without admiring this beautiful dispensation of Providence, extending over so vast a scale of being, and demonstrating the unity of plan on which the whole system of organized creation has been devised.

5. Return of the Sap. 32 032 033 034 035 036 037 038
6. Secretion in Vegetables. 38 038 039 040 041 042 043
7. Excretion in Vegetables. 43 043 044 045 046 047

[440 - Early concept of "recapitulation"] It is remarked, in farther corroboration of these views, that the animals which occupy the highest stations in each series possess, at the commencement of their existence, forms exhibiting a marked resemblance to those presented in the permanent condition of the lowest animals in the same series; and that, during the progress of their development, they assume, in succession, the characters of each tribe, corresponding to their consecutive order in the ascending chain; so that the peculiarities which distinguish the higher animal, on its attaining its ultimate and permanent form, are those which it has received in its last stage of embryonic evolution. ... [443] Nor is the human embryo exempt from the same metatnorphoses, possessing, at one period, branchiae and branchial apertures similar to those of the cartilaginous fishes, a heart with a single set of cavities, and a brain consisting of a longitudinal series of tubercles; next losing its branchiae, and acquiring lungs, while the circulation is yet single, and thus imitating the condition of the reptile; then acquiring a double circulation, but an incomplete diaphragm, like birds; afterwards, appearing like a quadruped, with a caudal prolongation of the sacrum, and an intermaxillary bone; and, lastly, changing its structure to one adapted to the erect position, accompanied by a great expansion of the cerebral hemispheres, which extend backwards so as completely to cover the cerebellum. Thus does the whole fabric arrive, by a gradual process of mutation, at an extent of elaboration and refinement, which has been justly regarded as constituting a climax of organic development, unattainable by any other race of terrestrial beings. ... It must, I think, be admitted that the analogies, on which the hypothesis in question is founded, are numerous and striking; but great care should be taken not to carry it farther than the just interpretation of the facts thethselves may warrant. It should be borne in mind that these facts are few, compared with the entire history of animal development; and that the resemblances which have been so ingeniously traced, are partial only, and fall very short of that universality, which alone constitutes the solid basis of a strictly philosophical theory. Whatever may be the apparent similarity between one animal and another, during different periods of their respective developments, there still exist specific differences, establishing between them an impassable barrier of separation, and effectually preventing any conversion of one species into another, however nearly the two may be mutually allied. The essential characters of each species, amidst occasional varieties, remain ever constant and immutable. Although gradations, to a greater or less extent, may be traced among the races both of plants and animals, yet in no case is the series strictly continuous; each step, however short, being in reality an abrupt transition from one type of conformation to another. In many instances the interval is considerable; as, for example, in the passage from the invertebrate to the vertebrated classes; and, indeed, in every instance where great changes in the nature and arrangement of the functions take place.