heart, like every living muscle, is charged with force, not motor or contractile, but chemical. The chemical affinity of its elements, resisted by vital or nutritive. action, accumulates within it, creating a state of tension and proneness to action, precisely such as exists in the Leyden jar. The comparison is just, though incorrectly used. Muscular contraction from a stimulus is the analogue of the electrical discharge by means of metallic contact, in which the resistance is removed; and the spontaneous contraction of the heart is parallel to the spontaneous discharge which ensues when the resistance is too weak.

An adequate account of the facts appears to be conveyed by the following statement. In the muscular structure or nervous ganglia1 of the heart, the chemical and vital forces are so balanced, that they assume a state of alternating activity. It might be said that the vital force exists in large quantity, but of low intensity. Hence, when, by the process of nutrition, the chemical affinity has been accumulated to a certain amount, it overpowers the vital resistance, and that chemical change. which is the cause of contraction ensues. And the same series of changes continually recurs, because the vital state is constantly renewed. It is possible that the maturity of the cells which constitute the muscular fibre, being accompanied by a failure of their vital power, may give the occasion for the ascendency of the chemical force; but the phenomena of voluntary muscular contraction, and the fact that the heart's action is often more rapid in proportion to the debility of the vital power, seem opposed to such a view. The action may be roughly compared to the alternate formation and

1 There are many circumstances which favour the idea that the action of the heart is dependent upon the ganglia contained in its substance.

decomposition of the ammoniuret of mercury in the course of an intermittent electric current.

In the foregoing remarks, it has been assumed that the vital force is characterised by a varying intensity of action. In proof of this law, it is sufficient to refer to the normal succession of the sleeping and waking states. The heart may be said to wake and sleep with each recurrence of its beat.

With regard to the mode in which chemical change of the muscular tissue effects its contraction, nothing certain is known. There is no difficulty, however, in the conception of such a causal relation, since the production of mechanical force by means of chemical action is one of the most familiar of facts, and the muscular structure may, without any violence, be regarded as a mechanism adapted for the development of mechanical effects from slight changes of composition.

III. With regard to the process of secretion, there is ample evidence that it depends upon a modified exertion of the chemical affinities. The following facts may be referred to:

1. The lower composition of the secreted fluids. In the case of the great mass of the secretions, including those of a nutritive character (as the milk), this less vitalised constitution is evident, and the seminal fluid, there is reason to believe, is no exception. To what, e.g., but an exercise of chemical affinity can the formation of sugar by the liver be referred?

2. The dependence of the secretive action upon the same stimuli and general conditions as the other functions, and especially upon the nervous force.

3. Its promotion by the local application, or presence in the blood, of medicinal or other substances, the influence of which cannot increase, but must tend to diminish the

vital resistance of the organs. It is not unlikely that in some instances the secretive action is normally maintained by the decomposing influence upon the gland tissue of substances, themselves in a state of decomposition, circulating in the blood.

4. An over-stimulation of secretion leads directly to destructive and anti-vital changes. Thus, as Mr. Paget has observed, the first stage of inflammation appears to be merely an increase of secretion. Salivation runs on to ulceration. One effect of destructive agents applied to the surface of the body, as a burn, severe pressure, or chemical irritants, is to induce secretion.

5. Professer Graham has rendered it probable that the passage of osmotic currents through animal membranes. is dependent upon slight decomposing changes taking place in them.

6. Secretion may continue after death, being then analogous to the post-mortem contraction of the muscles.

The production of electricity and of light must be enumerated among the animal functions, but it will be sufficient merely to allude to them. There is no cause to which they can be referred with more probability than to chemical changes in the electrical and luminous organs. And the generation of electricity is known to be determined, like the other functions, by any stimuli which tend to overthrow the chemico-vital equilibrium, either in the organs themselves, or those portions of the nervous system which supply them.

The view of the vital functions advocated above has many important bearings upon special branches of physiology and pathology, which cannot now be enlarged upon. The great advantage which seems to result from it is the simplification it effects in the conception of the vital force itself. One whole division of what under other

views is considered as vital action, being thus transferred to the domain of chemical agency, the idea of the vital force stands out clear and distinct before the mind as the peculiar molecular action which forms and nourishes the living body. That is its nature; that its entire scope. Thus, by resistance, it accumulates chemical force, and furnishes the conditions under which THE FUNCTIONSmotion, nervous action, secretion-exhibit themselves as the results of chemical affinity.

And the idea of the animal body, the fundamental conception or plan on which it has been framed, appears to be simply that on which we ourselves act when we wish to construct a machine. We use one modification of force as a resistance to another, privileged herein with the power of imitating, at an infinite distance indeed, the sublimest of the material works of the great Creator of all things.

And further still, this view of life opens to us yet another indication of the unity of principle that binds creation into one. On earth we see the antagonism of two forms of force yielding a well-nigh boundless variety of beautiful, useful, and happy action in the successive grades of animal existence. In the heavens, the antagonism of two forms of force develops the regular motions of the planets, and constitutes the law which ordinates the universe.1

1 I have perhaps failed to indicate with sufficient clearness that the production of functional action by chemical change depends upon the mode in which such change takes place. It is not every decomposition in the living body that necessarily results in a function, but such changes only, and changes of such intensity, as are adapted to act upon the functional mechanism. In a steam-engine it is not every possible expansion of the steam that causes a revolution of the wheels, but only an expansion which takes place in a sufficient and yet limited degree, and in a special direction. In the animal, passive decay of the tissues, as of an unused muscle, and excessive decay, as in some forms of disease, do not cause, but abolish, function.

Biol

Morphology.

XXIV.

ON PHYSICAL MORPHOLOGY, OR THE LAW OF ORGANIC FORM.

1858,

IN studying the development of the mammalian ovum my attention was struck by the form in which the laminæ dorsales make their first appearance. The layer of cells which constitutes the germinal membrane being completely formed, and separated at one point from the enclosing membranes, the lamina dorsales rise up in this portion as two parallel ridges or folds. The thought suggested itself to me that interstitial increase of the germinal membrane, under the limiting influence of the external capsule of the ovum, must result in a folding of the membrane upon itself just in some such manner. If a flexible layer increase in length while its ends remain at the same distance from each other, it is wrinkled up; by laying a handkerchief on a table, placing the hands firmly upon it at a short distance apart, and gradually approximating them, such folds may be produced.

The idea thus suggested to my thoughts led me to further investigation, and many instances soon presented themselves in which the forms assumed by developing structures seemed at least to be distinctly traceable to the mechanical conditions that were present. The law which prevails so generally in the vegetable world, that