CILIA. 



637 



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 which the appearance of the 

 moving cilia has been mistaken for a circu- 

 lating fluid,* or ascribed to other causes than 

 the real one, and their existence erroneously 

 denied. 



Secondly, It is well known that in cold- 

 blooded animals the blood continues to move 

 in the capillary vessels for some time 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 contractility, 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 not im- 

 possible, for example, that a certain degree of 

 agitation may be occasioned in the blood by 

 the elastic resilience 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 the circu- 

 lation, for the blood does not necessarily pre- 



* As by Baker, Guillot, and others. 



serve its original course, nor indeed any con- 

 stant direction. (See CIRCULATION.) 



Thirdly, In several plants motions have been 

 observed in the fluids which are contained in 

 their cells or vessels in determinate directions, 

 and seemingly independent of any contraction 

 of the parietes of the containing cavities. The 

 best known example of this is in the Chara. 

 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 in 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 experi- 

 ments of different observers on this point being 

 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 contractions 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 the Animal 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 will find his labour 

 rewarded by much curious and interesting 

 discovery. 



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 *Ant. de Heide, Anatome mytuli, &c. 8vo. Amst. 

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 Bat. 1695-1719. * Baker, Of microscopes, &c. 

 8vo. Lond. 1785. Hales, Haemastaticks, 3d edit. 

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 belustigungen, vol. iii. 4to. Niirnbcrg, 1755. *Spal- 

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 *O. F. Miitler, Hist, vennium terrestrium et fiuvia- 

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