636 



CILIA. 



same regular order for a time, and in some 

 animals for a long time, after death or sepa- 

 ration from the body; and it is remarkable, 

 although perhaps we are not warranted by ob- 

 servation to lay it down as a general rule, that 

 there is a correspondence in the duration of the 

 ciliary motion after death and the persistence 

 of muscular irritability. In the Tortoise, for 

 instance, in which it is well known that the irri- 

 tability of the heart and other muscles endures 

 remarkably long after death, the ciliary motion 

 is also of extremely long continuance ; while 

 in Mammalia and Birds, the ciliary motion and 

 muscular irritability are both comparatively 

 soon extinguished. 



On the whole, therefore, without laying any 

 stress on the alleged discovery of a muscular 

 apparatus by Ehrenberg and the other authors 

 mentioned, we may venture to conclude that 

 the facts known respecting the motion of the 

 cilia are all reconcilable with the opinion that 

 it is produced by muscular contractility. 



10. Strange as it may seem, after what has 

 been said, some observers maintain that the 

 cilia have no real existence, even in cases 

 where the appearance of them is the most 

 perfect, and that the whole is an optical de- 

 ception. I allude particularly to Raspail ; 

 according to him the water which quits the 

 respiring surfaces has, in consequence of the 

 change produced in it by respiration, acquired 

 a different density, and consequently a dif- 

 ferent refractive power from the surrounding 

 fluid ; it therefore produces the appearance 

 of lines or streaks at the surface of the parts, 

 which streaks are the supposed cilia. It is 

 scarcely necessary to repeat that the cilia are 

 seen when at rest, when all motion of the 

 water has ceased, and that they are evident in 

 circumstances in which no interchange of ma- 

 terials can take place between the tissue and 

 the water in contact with it; and indeed, after 

 the details already given, it is needless to say 

 more in refutation of this view. 



1 1 . Of the motion 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 when 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 mag- 

 nitude and velocity of the current seem to be 

 proportionate 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 fluids 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 alleged 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 Sertulariae. 

 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- 

 gation, 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 Sertulariae. Indeed both 



