growth, of its decay, and the intermediate one of its functional activity, which is dependent upon the first, and which causes the third." 1

"We may look upon the death of such cells (the muscular tissue), whose term of life might otherwise have been considerably prolonged, as the result of the expenditure of their peculiar modification of force under the guise of mechanical power." 2

In this representation it appears to me that the relation of cause and effect is inverted-that the existence of a controlled and subjugated tendency to chemical change in living bodies is the origin of all the capacity for functional action which they display, and that the disintegration of their tissues is not a "result" or "condition" of their activity, but rather the moving spring and source of that activity itself.

The life of the body being one, its functional power must be one also. Widely as they may differ in their immediate form and object, the functions, when regarded in relation to their origin, may not be isolated from each other. They are common products of a single power, which requires to be investigated at once in all its modes of action. Hence probably the want of success which has attended the various attempts that have been made to trace the physical causes of separate functions. But, on the other hand, much of the obscurity which attaches to the ideas of life and the vital force appears to have arisen from the attempt to include under one denomination, and to refer to one mode and development of force, phenomena of diverse, and indeed opposite characters.

Broadly as the line of demarcation is drawn by nature

1 Dr. Bucknill, British and Foreign Medico-Chirurgical Review, No. 29, P. 226. 2 Dr. Carpenter, Human Physiology, p. 109.

between those processes by which the living organism is built up and maintained, and those which involve the death and disintegration of the tissues in which they occur, the prevailing tendency of physiological speculation has been to include both series of actions under one name, and to refer them to the immediate operation of a common power. They have been termed indiscriminately vital actions, and adduced without distinction as instances of the direct operation of the vital force.

Thus Liebig says: "The active or available vital force in certain living parts is the cause of the mechanical phenomena in the animal organism." 1

And Dr. Carpenter thus expresses himself: "The contraction of any muscle upon the application of a stimulus must be attributed to an exercise of vital force engendered by previous acts of nutrition." 2

And again, speaking of muscular and nervous action, he says: We are entitled to affirm that each is a peculiar modus operandi of the same force as that which is concerned in cell-formation."

"3

According to this view, the vital force is made the direct agent in actions essentially different. Hence arises the impossibility of defining it; for while the words are so used it is surely in vain to seek to attach to them any signification more definite than that of a general expression for all the changes which take place in a living body. Any term similarly used would become equally obscure and unsettled. By thus including in one category actions so opposed as function and nutrition, the phenomena of life are placed in an attitude of irreconcilable variance with those which pertain to all other branches

1 Organic Chemistry of Physiology and Pathology, p. 221.

2 Human Physiology, third edition, p. 476.

Philosophical Transactions, Part 2, p. 737. 1851.

of physical science. The fatal error has been to overlook the fact that two forces (or modes of force) are at work in the living body. It has not been perceived that the chemical affinities of the animal organs constitute a source of power co-equal with, and precisely measured by, the power of the vital force. The work of two agents has been assigned to one. If now the omission be supplied, and the vital and chemical forces be recognised as the two forces of organised matter-the former as the resistance, the latter as the resisted force, and therefore the force available for action—a large part of the obscurity which envelopes the theory of vital action is at once removed. An uniformity of principle is seen to prevail between the laws of the organic and inorganic worlds, and the facts hitherto so intractable arrange themselves without difficulty in accordance with some of our most familiar conceptions.

Bearing in mind that no explanation is offered of the nutritive processes in the living body, it will be seen that upon the theory propounded there is a perfect analogy between the animal body and a self-acting machine.

In both there exists a mechanism adapted to the performance of certain defined actions, and a reservoir of power or force by which that mechanism is kept in operation. In both, the source of this power is essentially the same. In living bodies one tendency of matter, its chemical affinity, is held in check; in any machine that is to manifest a capacity for action, art must bring into a like condition of restraint some tendency of matter, either the same or similar.

In the simple instance of a clock, the gravitation of the weights, controlled by an adapted mechanism, is the power which effectuates its functions-the revolution of the hands, the striking of the hour. In the watch, the restrained elasticity of the spring holds the same rela

tion. The steam-engine owes its power to the repressed expansiveness of the vapour. There is no instance, indeed, of an artificial accumulation of force or capacity for action that does not depend upon this principle. Matter restrained from the fulfilment of any of its natural tendencies affords power; the removal of the restraining force, permitting the play of the tendencies so controlled, produces action; which action may be made to subserve any purpose by suitable modification of the resistance, and the employment of an adapted mechanism.

In this respect the organic and inorganic worlds obey a common law. Organisation gives capacity for action. only by virtue of the resistance it presents to the chemical forces; these chemical forces, acting under definite limits, and in connection with various structures, being the true sources of all functional activity. A living body is a divinely-made machine, constructed, indeed, with a marvellous delicacy, perfection, and complexity, and depending upon a power, the vital modification of force, which it is wholly beyond our skill to imitate or comprehend, but still involving in its working no other principles than those which we every day apply, and see to regulate the entire course of nature.

For the inorganic world furnishes abundant instances of the same balancing of forces resulting in a similar activity or capacity for action. The term irritability, in so far as it denotes a capacity for responding to stimuli, confined hitherto to organised structures, might with perfect accuracy receive a more extended application. It exists in whatever form of matter there is found the same powerful tendency to change of state with which it is associated in living bodies. Thus, in the chloride or iodide of nitrogen the slightest touch induces an explosion. In the case of gunpowder, the tendency to change in which

is less energetic, the chemical affinities of the materials are brought into action by the momentary application of intense heat. In the same way a solution of certain salts, when the cohesive force is barely counterbalanced by the solvent power of the water, will assume the crystalline form upon the gentlest touch, or the mere passage of a vibration. The slightest scratch causes unannealed glass to break.

In these instances and very many more might be adduced-it is surely correct to say that action ensues on the application of a stimulus; and in them all it is obvious that the action is immediately due to pre-existing and restrained tendencies to change of state. The stimulus is only in a secondary sense the cause of the phenomenon, and evidently determines it by removing the condition which forbade the previous operation of those tendencies. In all such cases the modus operandi is the same as that of the mechanisms previously referred to, and they are precisely analogous to the simpler contrivances in which a suspended weight is made to fall upon the disturbance. of its equilibrium by slight causes.

If the doctrine of the correlation of the physical forces be applied to material actions or changes of this class, it becomes at once apparent that the correlated force is neither the resistance nor the stimulus, but the controlled or latent tendency to change.

Thus, e.g., the application of a certain amount of heat to a magnet suspends its attractive power. If, therefore, to a magnet sustaining a mass of iron sufficient heat be applied, there results an action-the fall, namely, of the iron to the earth, the cause of this action being the gravitation which the magnetic force had previously been exerted in controlling. It might be said that the gravitation is converted into motion; it would never be pro