operation of organic heat. Where indeed a vessel is more dilated, there we may suppose molecular changes, which will necessitate a further dilatation, by which additional quantities of fluid will be attracted from neighbouring parts; for although changes take place cotemporaneously in vessels which are contracted and comparatively empty, yet these lesser phenomena will be absorbed and counteracted by the greater. For the same reason, also, that the more expanded portions should, in the first instance, be the seat of a more vigorous extrication of heat and consequent expansion, by which currents are determined towards them, so also we must grant the possession of the same faculty when they have become more expanded by absorption. Once set up indeed, the process of expansion must go on in these parts, for anything we know to the contrary, either until a sufficient resistance is opposed by the pressure of neighbouring parts, or until the vessel is ruptured-for the coats will be less able to resist the distending force as the expansion progresses.

The stage of contraction in the laticiferous web which supervenes upon dilatation would seem to argue the abstraction or suspension of force, and such may be the case. This will appear if we leave those focal centres which have attained their maximum degree of enlargement, and cast our eyes upon other portions of the same system of vessels. There we find irregularity in the sizes of the tubes: and hence we may begin

again as at first; and as the absorbing power of the original centres is suspended we may argue that the vessels which are most dilated at the time of suspension, will in their turn take upon themselves a faculty of expansion and become centres of attraction. These secondary centres will expand at the expense of the neighbouring vessels: and this process will go on (as in the first instance) until a barrier is opposed to further progress by external resistance. Cotemporaneously with the cessation of the flow of latex to these points a third order of centres will be established in other parts of the web where the vessels are largest, and there being no impediment to their enlargement, they will go on dilating, until, as in the former instances, the efficient cause is balanced by external resistance. A fourth order of centres will succeed the third, and a fifth be superadded upon the fourth, and so on in an unending series, the position of the new centres in each case being in those portions of the vascular web which are most dilated when the old centres cease to enlarge.

In the absorption produced by the expansion of these fugitive hearts we find a cause which will tend to empty all congested portions of the vascular web, and by this means we may partially account for the supervention of the stage of contraction upon that of dilatation. A second cause may also be found in the changes which have taken place in the contents of

the dilated centres, for these contents may be supposed to be impoverished by having subserved to the nutritive wants of the part, and to have become on this account far less fitted to the performance of those molecular changes which originate heat: and hence the stage of contraction may be in part due to a direct diminution of the local manifestation of organic heat.

Other agencies, without doubt, have some share in the determination of these phenomena, as we shall see in the sequel; but in the operation of heat, we find— as it seems the first step in the explanation of the primary absorption of the latex into the contracted vessels, and of the subsequent oscillation of this fluid to and from certain focal centres, while at the same time we are able to account, by the same means, for the change of position in the centres themselves.

II. OF THE MOVEMENTS IN SIMPLE CELLS, AND

1. Of extra-organic force as the agent in these

movements.

The functions of the laticiferous vessel and the common cell are called into action at the same time, and the season which determines or suspends the vascular changes determines and suspends also the growth and internal movements of the cell.

Whatever may be the cause of the appearance of the primary germs or nuclei in the organic fluids, we know that their molecular constitution is closely allied to the substance which forms the coats of cells and vessels, and also that heat is necessary to their future development. In these bodies, therefore, we may argue that expansion is inevitable under the operation of the last-named agent, and thus it is possible to conceive that the enlargement of growth may be no mysterious property but only the natural result of the operation of heat upon the substance. Even the formation of the cavity, by which the originally solid nucleus is converted into a hollow vesicle, may also be a part of the same process, for under the operation of an expansile force the particles of these minute fabrics will tend outwards from the centre.

When the cell is once formed we may readily understand that heat may be the cause of further development, for the material of which it is constituted will be acted upon like the coats of the laticiferous vessel. The process of rupturing which takes place in many of the free cells of organic fluids may also be the natural consequence of unrestrained expansion, and this result would probably be constant if in many cases the process were not kept within bounds by collateral pressure, as is the case in the cellular elements of solid tissues.

In the coats of simple cells there do not appear to

be any periodical changes corresponding to those witnessed in the milk-vessels; but, at the same time, there is a fact which may be construed to indicate the existence of obscure changes in some measure analogous. The concentric layers which occupy the interior of many cells would seem indeed to refer to periodical alterations, which may be understood to be possible enough. We may suppose, indeed, a cell to submit to a state of contraction after having attained to its maximum state of dilatation, partly because its contents have been abstracted by neighbouring cells which are in process of development; and partly because the same contents are less fitted to the generation of heat after having subserved in some measure to the nutritive wants of the part. And further, we may suppose a second state of expansion during a period of comparative inactivity in neighbouring parts, which itself might result in contraction when the contents were wasted by nutrition, or wanted for the growth of other cells.

Under similar circumstances, also, the same processes might be repeated a number of times. In each case we may suppose the formation of one of the concentric layers, which line the interior of the cavity, to mark the epoch of nutrition, activity, and dilatation ; and in this way the layers may be regarded as relics which give evidence of slow periodical changes in these cells, analogous, in some degree, to the more realized