leaves recover from the closed and pendent condition in which they are found at night, and—if not disturbed in any manner-they remain erect and unfolded until evening, when they again close and droop; and these alternations recur in this order so long as the characteristic sensitiveness is retained. In both cases, indeed, there is a manifest connexion between the phenomena and certain changes in the relative positions of the earth and sun, the one referring to the annual, and the other to the diurnal revolution; and hence it may be argued that the vital movements of the sensitive plant are due to the joint operation of cosmical force, and of an innate principle which belongs to the individual organism.

The egg of the lizard, like the seed of which we have spoken, exhibits no signs of development unless it be quickened and fostered by external agents; and the same aid is necessary after the animal has escaped from the shell. In the perfect state there is a sensitive and intelligent principle by which the actions are regulated and governed, but this does not supersede the foreign powers which acted upon the egg. It is a constant rule, indeed, for this animal to be active and full of life in warm weather, and to hybernate in cold -the periods of animation and torpidity being in exact correspondence with the summer and winter; and it is equally constant for the same animal, when in its active state, to wake throughout the day and to

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