sleep throughout the night;-and hence we may argue that the vivifying rays of the sun are essential to the continuance as well as to the commencement of existence, and that the vital movements of the lizard depend, not merely upon a source and spring of action that is peculiar to the creature itself, but also upon a more general and comprehensive agent, which affects at one and the same time the entire realm of nature.

As in the sensitive plant and lizard, so in other plants and animals, the vital movements are found to refer partly to internal and partly to external agencies. In some cases there may be greater energy in the forces which reside within the organism; in others the influence of surrounding nature may be more powerful; but in every case the two classes of agencies are invariably associated.

In order, therefore, to arrive at any satisfactory knowledge of vital motion, it is necessary to examine it in relation to cosmical as well as to organic force,or rather in relation to the forces which originate within and without the organism--for we find, on the one hand, that cosmical force pervades the organism and constitutes part of its vitality, and, on the other, that organic force is not restricted to the limits of the body. Before we enter upon any special and systematic enquiries, however, it may be well to endeavour to obtain some preliminary information upon the mode in which the bodily structures are affected by some of

the more ordinary varieties of what may be called extra-organic force, in contradistinction to that which, originating within the body, may be called intraorganic. And this shall be the subject of our present chapter.

As we might anticipate, the parts of the plant or animal which respond most readily to extraorganic agents, are those which are simple and rudimentary in their character; for in the higher textures, such as muscle, the influence of this class of agents is greatly masked by the workings of inherent vitality. In the simple tissues, however, there is no obscurity.

In the sensitive plant, for example the cellular cushions which move the leaves are found to expand under the influence of light and warmth, and to contract on the withdrawal of these agents; and with few exceptions, and these but apparent, the same law is observable in all similar organs.

The simpler fabrics of animal bodies, which are furthest removed from the control of the nervous system, and in this respect most nearly allied to the tissues of the plant, are also found to move in obedience to an external impulse. The subcutaneous areolar web is shrunk and puckered in winter, so as to give rise to the appearance called "cutis anserina,"

and it is relaxed or expanded in summer. When exposed to cold, also, the dartos contracts in a very remarkable manner, and an opposite condition is induced by the operation of warmth. And so likewise in the vascular coats of erectile tissue.

These instances furnish different points of view, from which we may regard all the principal forms of rudimentary tissue. The irritable tumours of the sensitive plant consist chiefly of cells which do not differ from the ordinary cells of plants. Areolar tissue is composed of fibrils of various sizes, which are formed from the same material as the primary cells, and which material appears to be absolutely identical in plant and animal. The tunic of the dartos is only another form of areolar web, in which there is a preponderance of certain thicker fibres, that are present everywhere in less abundance; and between these fibres and those which enter into the composition of the vascular coats, there is a direct and immediate gradation. Each form, indeed, is transitional to the other, and the whole constitute a group that includes all structures subordinate to true muscle. It is of moment, therefore, that we find these tissues to contract when exposed to cold, and to pass into an opposite state under the influence of warmth, inasmuch as this action must be supposed to be common to all the rudimentary tissues of the economy. In this way,

then, we obtain evidence of the operation of external agents in the phenomena of vital motion, and learn at the same time the mode of action, so far at least as that force is concerned which is most intimately wedded to motion—namely, heat.

This result is in harmony with that which marks the operation of the same agent in inanimate bodies, for these contract or expand according to the presence or absence of heat: and yet there is an important difference in the midst of this seeming analogy. It is found, indeed, that there is a much greater alteration of volume in organic solids, under trifling changes of temperature, than in any other bodies. Under no higher degree of heat than what is expressed by the term warmth, the fibrils of the areolar covering of the body are greatly elongated, provided they were previously in a contracted state; and in the fibres of the dartos and erectile tissue, the former length may be doubled. In inorganic solids, on the contrary, the change, under these circumstances, would be almost inappreciable; and even in fluids, which are apt to undergo greater changes than solids, and are very sensible to the motive power of heat, as may be seen in the mounting or falling of the liquid in the tube of the thermometer, we are acquainted with no instances where the change is in

any way commensurate with that which is witnessed in these organic solids.

Notwithstanding this peculiarity, however, it is evident that the simple vital movements of which we speak are connected with, and in some degree determined by, ordinary heat, and on further examination we find no reason to doubt this conclusion.

It is found, indeed, that various inorganic solids are affected differently by heat, some being acted upon to a much greater extent than others: and this is sufficient to show that there may be a still further difference in organic substances, without any departure from the mere physical character of the phenomenon.

It may be observed, also, that the solids in question are composed of several elements, as oxygen, hydrogen, nitrogen, and carbon, which for the most part remain gaseous when in an uncombined state, and of which the mutual affinity is so feeble that they readily pass into an aëriform condition,—and hence it may be supposed that such solids will undergo a greater change of volume under the action of heat than an inorganic substance, in which the constitution is simple and the molecular affinity more stable. And for the same reason it need be no ground of wonder that the change should be greater than in water, or in any other fluid whose particles do not tend to