Secondly, It is well known that in cold blooded animals the blood continues to move in the capillary vessels for some thne after the heart has been cut out. This motion for the most part goes on at first steadily from the smaller to the larger vessels in the arteries as well as the veins, and afterwards becomes oscillatory. Haller, who particularly investi gated the phenomenon, was of opinion that it could not be attributed to contraction of the large vessels, to gravitation, nor to capillarity; he therefore attributed it to some unknown power which he conceived to be exerted by the solid tissues on the blood and also by the glo bules of blood on each other, and to this power, until farther investigation should eluci date its nature, he gave the name of attraction. The same opinion or a modification of it has been taken up by succeeding physiologists ; accordingly many maintain the existence of a peculiar propulsive power in the coats of the capillary vessels different from contmctility, or that the globules of blood are possessed of the power of spontaneous motion. Among others, Dr. Alison has adopted and extended this view in so far as he regards the motion of the blood in the capillaries as one of the effects produced by what he calls vital attraction and repulsion, powers which he conceives to be general attri butes of living matter, or at least to manifest themselves in other processes of the living economy -besides the capillary circulation.
The motion in question has certainly not been as yet satisfactorily accounted for by re ferring it to the operation of known causes. At the same time we can scarcely admit that the influence of such causes has been wholly avoided in the experiments in which the phe nomenon has been observed. It is nut im possible, for example, that a certain degree of agitation may be occasioned in the blood by the elastic yesilience of the vessels reacting- on it, after the distending force of the heart has been withdrawn. The necessity of the case there fore, though great, seems scarcely such as alone to warrant the assumption of a peculiar attrac tive or repulsive power acting on the blood at sensible distances, of whose existence in the animal economy we have as yet no other evi dence. It may be remarked, finally, in regard to the phenomenon alluded to, that it cannot properly be termed a continuance of tbe circthr lation, for the blood does not necessarily pre serve its original course, nor indeed any con stant direction. (See CiltCULATION.) Thirdly, In several plants motions have been observed in the fluids which are contained in their cells or vessels in determinate diiections, and seemingly independent of any contraction of the parietes of the containing cavities.. The hest known example of this is in the Mira. Its jointed stem consists of a series of elon gated cells, which contain a clear fluid with globules suspended in it. The globules are moved up one side of the cell and down the other in continual circuit. No contraction can be perceived irt the parietes of the. cells, which
are indeed of a rigid texture, and this myste rious movement has therefore been ascribed to; some unknown and invisible impelling power. _ It is doubtful, however, whether the motion can go on unless the cell is entire, the experi7 ments of different observers on this point beinF contradictory, and it certainly has never been shewn that separated portions of the tissue continue to excite the motion. In this state of knowledge on the subject we can scarcely admit this or similar motions of vegetable juices as unequivocal examples of the opera tion of an impulsive power of the kind referred to; and even on the contrary supposition it does not follow that such a power exists in animals.
On the whole therefore, from what has been said regarding the Several examples adduced, we may conclude that they do not afford une quivocal evidence of visible motions being produced in fluids in the animal body, inde:: pendently of contmctions of containing solids or of the action of cilia; and, consequently, that viewed in reference to the ciliary motion, they form no adequate reason for doubting that the fluid is moved mechanically by cilia.
I may conclude this article by observing? that though the general existence of the ciliary motion in theAnimal Kingdom is already suffi ciently established, yet many particular in-" stances of it must still remain to be found out, especially in invertebrated animals ; and who ever has opportunities and inclination to cul tivate this field of inquiry find his labour rewarded by much curious and interesting discovery.
partments. The first, of these will compre hend a description of the course of the blood in man ; the second of its Course in animals. In the third will be considered the phenomena presented by the blood during its motion, the properties of' the organs in which it circulates, and the powers by which it is propelled ; and in the fourth will be mentioned the more im portant circumstances connected with the other functions which modify the circulation.
The term circulation applied by its cele brated discoverer, Harvey, to the motion of the blood, is sufficiently expressive of the general fact that this fluid, or the greater part of it at least, in being carried through the body, moves in a circular course, or, that in performing its jour ney through the body, the blood always re turns to the same place from which it set out. The term is equally applicable to the func tion by which a supply of nutritious fluids is kept up in the lowest animals, in which a pro gressive motion of a fluid of the nature of blood takes place, as well as in the highest ; for in nearly the whole of them there is a central part of the circulatory organs, which forms the rallying point, as it were, of the rest,-from which the blood begins its course and to Which it is brought back, in a longer or shorter period of time, after having passed through the dif ferent organized parts.