exaggerate the original expansion, and so give rise to the phenomena of inflammation; but under ordinary circumstances, the "capillary power" is so ruled, that all parts are affected equally, the office being to destroy the contraction of the vessel, which would counteract the action of the heart, and not to determine the flow of blood to one part rather than another.

The mode of capillary action may indeed be illustrated by the circumstances which favour the injection of the vessels, which is necessary for anatomical purposes, and the illustration is perfectly legitimate. The fluid, as we know, passes from the syringe into the minutest vessel with great readiness, so long as the animal continues warm, or when the warmth is restored by immersion in the warm bath, but the contrary is the result when the animal is cold. In the one case the operation succeeds, as it would appear, because the passages present little or no resistance to the force of the syringe-in the other it fails, on account of that contracted state which has taken place in the vascular tunics, and in the tissues generally, when the body has become cold. No other explanation is admissible than that which refers to physical agents for life has ceased;-nor is any other needed, for all the phenomena may be accounted for by the changes which must take place in the coats of the vessels, under varying degrees of temperature.

So also, we may argue of the natural circulation, if

the heart be substituted for the syringe, and we may suppose that the blood moves sluggishly when the vessels are contracted, and rapidly when this cause of resistance is overcome, and the calibre expanded, and that the so-called capillary power is directed to the removal of impediments to the action of the heart, and not an inherent heart-like faculty of action.

II. OF THE CELLULAR MOVEMENTS IN ANIMAL

BODIES.

Great attention has been paid to the anatomical peculiarities of cells, and to their modes of growth, but this has been almost altogether without reference to efficient causes. Indeed, in this case, as in many others in physiology, these important considerations have been neglected on the ground that they referred to some principle whose operations were in no degree cognisable to the reason.

On comparing, however, the analogous sections in the history of the plant and the animal, we find many reasons to suppose that the cells in each case are acted upon by the same agents, and in the same manner. In the animal, external warmth is necessary to growth, as is evident from the history of nutrition in hybernating animals, for in them the process of growth is arrested during winter. Light, also, is necessary, as we may see particularly in the effects of this agent in determining the free development of the blood and the

E

coloured cells of the epidermis. On the other hand, the history of plethora, when contrasted with the opposite effects of anæmia, may serve to show that free nutrition is associated with an abundant development of organic force.

It would appear, also, that there are phenomena in animal bodies which seem analogous to "rotation" (of the vaguer sort,) and that these are referrible, in part at least, to physical agencies. Dr. Addison, indeed, pointed out some time ago the existence of moving molecules in many forms of free cells, as in those of mucus, pus, and lymph, and also in the colourless cells of the blood; and I have heard him state, that after watching these movements until they had ceased, he has repeatedly seen them renewed and made as brisk as ever by accidentally placing the watch-glass in which the fluid had been contained upon the warm mantelpiece. It would thus appear that the process of cooling after removal from the body had arrested their movement, and that they were renewable by warmth, and in this manner we may detect further proofs of the continued operation of these agents.

It appears, also, that the movements of "vibratile cilia" continue so long as the supporting cells retain their integrity and are acted upon by warmth, and that they cease in opposite conditions; and this being the case we may obtain some light as to the cause of these singular phenomena. To find any proof of ex

ternal agencies, indeed, is to make a discovery by which we are relieved from supposing the existence of any mysterious intelligence in this organic rudiment, by the workings of which the cilia are moved; and when we escape from this notion, then there is no difficulty in supposing a continued series of inductive charges and discharges between the ciliate-cell and any other bodies which may have a share in this action, and in this way the vibration of the cilia may be considered as the intelligible sign of these charges and discharges.

CHAPTER III.

B. OF VITAL MOVEMENTS SUCH AS ARE SEEN IN THE IRRITABLE TUMOURS OF THE SENSITIVE PLANT, IN THE COATS OF THE ALIMENTARY CANAL, AND IN VOLUNTARY MUSCLES.

THESE phenomena are arranged in two sections, according as they belong to the animal or vegetable kingdom of nature; and this division furnishes the primary heads of the present chapter.

SECTION I.

OF MOVEMENTS SUCH AS ARE SEEN IN THE IRRITABLE TUMOURS OF THE SENSITIVE PLANT.

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

movements.

Like all other phenomena of plant-life, these movements point rather to extrinsic than to inherent agents, when they are examined in relation to their efficient