additions, each of them, as we presume, possessing every power or property of the other, together with what may be necessary for the exercise of those functions which belong to it exclusively. This community of properties might be expected to exist more particularly with regard to those nerves which are immediately connected with the brain or the spinal cord, which parts, from their connexion with each other, it is natural to suppose must possess, to a certain extent, the same properties. It has been found that the power of transmitting the galvanic influence, if not confined to certain nerves, is at least much more remarkable in some of them than in others, and this difference exists in so great a degree, that many eminent physiologists have been unable to excite any contractions in the nerves that are connected with the ganglia, and which are not under the control of the will. Yet if the nervous power be identical with galvanism, there appears to be no assignable reason, why these nerves should not be at least as sensible to this stimulus, as the nerves that belong to the voluntary organs, since they cannot be supposed to be deficient in the mere nervous functions, although they may not be possessed of those which are confined to the sensorium.

We have frequently had occasion to remark upon the great diversity in the nature of the substances which act as stimulants to the muscles; and although I have endeavoured to establish the doctrine of their independent contractility, yet, at the same time, it was shown that in a great number of in

stances the stimulating substances act through the intervention of the nerves; but if we are to regard the nervous power as identical with galvanism, it will follow that all these agents, mechanical, chemical, and vital, must operate through the medium of electricity, a supposition which appears quite repugnant to our ideas of their physical relations, and of the nature of electricity itself. And I may observe, that the difficulty, with respect to animals that are without a nervous system, is scarcely, if at all diminished, by supposing that the nervous power is identical with galvanism; for we have the effects that are ascribed to galvanism produced without the existence of the nerves, which are supposed to be the channels through which this agent is conveyed. Should we go a step farther and maintain that galvanism is capable of being transmitted through every part of the body, as well as through the nerves, we may indeed elude the present difficulty; but we become involved in the contradiction of supposing that nerves are necessary for the performance of a certain function, because they serve to convey the galvanic influence, yet that other parts of the body, as well as the nerves, are adequate to this conveyance.

Upon reviewing the subject of the theory of secretion, the inquiry, when considered in the abstract, may be stated as follows. We have a fluid possessed of certain properties, and consisting of certain components, from which various other substances are produced, the greatest part of them composed of the same elements with the primary fluid, but in different

proportions; by what means are these secondary substances formed? we may conceive of both chemical and mechanical agencies being concerned in these operations; if the substances produced are identical with any of the constituents of the primary fluid, or even very similar to them, it may appear probable that the operation is principally mechanical, whereas if the secondary substance differs considerably from any of the constituents of the primary fluid, we should naturally suppose that it has been produced by a chemical affinity, or by the combined effect of chemical and mechanical action. We must next inquire, how far the different modifications of chemical and mechanical actions, which may be conceived to exist, are sufficient to produce all the effects which we actually observe to take place, and to this inquiry we can only reply, that the present state of our knowledge on the subject of animal chemistry does not allow us to go farther than to say, that the changes which have been actually produced are very numerous, and that those which may be supposed possible are still more so, and that if there be any which we cannot explain, they will probably be equally difficult to account for upon any other principle. On this view of the subject, therefore, the question will rather be an appeal to our ignorance, than to any principle which can direct our judgment in deciding upon this point.

In the fourth place we must ask, in what manner are these supposed chemical or mechanical changes con

nected with the operations of the living system; which of the vital powers are called into action, and through what medium are they excited? When we consider the mere act of secretion, to which our present inquiry extends, I should say that contractility is the only vital power which is essential to the operation. The action of the heart, in the first instance, propels the blood into the capillaries; it is transmitted through these with different degrees of velocity, and subjected to various modifications of compression, so that its constituents are more or less intimately mixed together; some of its finer parts are transmitted into vessels too minute to admit of those that are more viscid or tenacious, while, at the same time, it may be supposed to experience various alterations from changes of temperature, from the action of the atmosphere, or from the mixture of the different secretions with each other. Then, although for the reasons stated above, I conceive that the action of the nerves is not essential to secretion, it is sufficiently obvious that the organs of secretion, in the higher orders of animals, are very much under the influence of the nerves, and are, in many cases, materially affected by them, so that we are in possession of an additional agent, by which we may multiply the number of possible combinations of elements, and produce a corresponding number of new substances. Still, however, we are to bear in mind, that we can form no clear conception of any mode in which the nerves can act upon the organs of secretion, except

through the medium of the circulating system, so that here again we reduce the primary operation to the contractility of the muscular fibre, notwithstanding the share which the nervous system may have in the effects, considered as a secondary agent.

A great difficulty which remains to be obviated, respects the formation of the saline secretions, or of those substances, the elements of which are not to be found in the blood, or at least not in sufficient quantity to account for the great accumulation that takes place in certain parts of the system, and where we are unable to point out any means by which they can have access to it. This is a difficulty which, I confess, appears at present insurmountable, and it can be said, that it attaches equally to every hypothesis that has been proposed; for it is at least as difficult to say in what manner the action of the nerves should produce lime from the blood, as how it should be produced by any operations of chemical affinity. To suppose that we are affording any real explanation of the phenomenon by ascribing it to the ope ration of the vital principle, or to any vital affinities, which is merely a less simple mode of expressing the fact, is one of those delusive attempts to substitute words for ideas, which have so much tended to retard the progress of physiological science.