11. Of the ?notion caused in fluids by the cilia.—One of the most remarkable characters of the motion produced in water and other fluids by the.ciliary action, is its definite di rection, which, except in some of the Infusoria, appears to be always the same in the same parts ; at least I have never been able to per ceive any exception to this rule. Appearances Would rather lead to -the belief that in the Infusoria the motion of the cilia is under the influence of the,will, which would account for this and other poSsible cases of exception.
We have hitherto taken it for granted that the currents in the water are owing to the mechanical effect of the moving cilia, without formally adducing proofs in support of the opinion; but at the same time the details already given must have served as such. The currents cease 3,vhen the motion of the cilia stops, they are strong and rapid when it is brisk, and feeble when it languishes; and though there are modifying circumstances or perhaps- exceptions, yet in general the mat7, nitude and velocity of the current seem to be rroportionate to the size and activity of the cilia. It is true that while doubts remained as to the existence of cilia in several well marked instances where the water unequivo cally received its motion from the surface over which it flowed, and, independently of any visible contractions of the animal tissue, there was also considerable room to doubt whether, even in the cases where cilia were manifest, the effect of these organs was wholly mecha nical, and whether the motion of the water was not rather due to some peculiar impulsive power in the tissue, differing from mechanical action. But more extended observation has almost wholly removed these exceptions, while it has considerably increased the number of conforming instances, insomuch that there seems at present no necessity for having re course to any other explanation of the motion of the flaids than that it is produced by the action of the cilia, and that their action is the result of muscular contractility, a known pro perty of animal tissues.
The phenomena of the ciliary motion seem therefore of themselves to afford no counte nance to the notion of a peculiar impelling power of the animal tissue, in virtue of which fluids are visibly moved along its surface, in dependently of impulse communicated to them mechanically by cilia or by contraction of in closing solids; nor am I aware of other facts Which either alone, or viewed in connexion with the former, warrant such a notion. But
as some physiologists believe in the existence of such a power, and found their opinion, at least partly, on alleg-ed examples of visible motions of fluids in organized bodies, pro duced without cilia and independent of con traction of the solids, it may not be amiss here shortly to consider the principal facts which have been adduced as instances of this kind.
First, Three cases have been already men tioned in which currents, more or less re sembling those produced by cilia, take place on surfaces on which cilia have not been de tected; these are the currents in the Sponge, those of the Tubularia indivisa, and those within the stem and branches of Sertularim. In regard to the Sponge, it is true that cilia have been diligently sought for and without success; still, considering the difficulty of the investi zation, it is not impossible they may exist in some part of the passages through which the water runs, though not yet discovered, espe cially as the ova possess evident cilia. With respect to the currents described by Mr. Lister within the stem of the Tubularia, it will be seen, on referring to the account of these, that farther observations would be 'required to settle the points here in question, viz. whether the floating particles receive their impulse from the surface over which they move independently of any contraction of the stem, and whether or not that surface is covered with cilia. To de cide these points satisfactorily it would be necessary to lay open the tube and make trial of detached portions of the tissue as in other instances. The same remark is in a great measure applicable to the currents in the stem and branches of Sertulariee. Indeed both instances have been described above only be cause of their seeming analogy with the rest, but further investigation is still required to determine their true nature. Neither these, therefore, nor the Sponge afford unequivocal examples of the peculiar motion of fluids al luded to taking place independently of cilia. Of course we may pass over without notice the cases in vvhich the appearance of the inovin„0- cilia has been mistaken for a circu lating fluid,*' or ascribed to other causes than the real one, and their existence erroneously denied.