CHAP, ii.] THE CONTRACTILE TISSUES. 165 



separate cilia of each cell, moved independently of the others, all 

 that would be produced would be a series of minute ' wobbles,' of 

 as little use for driving the fluid definitely onwards as the efforts 

 of a boat's crew all rowing out of time are for propelling the boat. 



Swift bending and slower straightening is the form of ciliary 

 movement generally met with in the ciliated epithelium of mam- 

 mals and indeed of vertebrates ; but among the invertebrates we 

 find other kinds of movement, such as a to and fro movement, 

 equally rapid in both directions, a cork-screw movement, a simple 

 undulatory movement, and many others. In each case the kind of 

 movement seems adapted to secure a special end. Thus even in 

 the mammal while the one-sided blow of the cilia of the epithelial 

 cells secures a flow of fluid over the epithelium, the tail of the 

 spermatozoon, which is practically a single cilium, by moving to 

 and fro in an undulatory fashion drives the head of the sperma- 

 tozoon onwards in a straight line, like a boat driven by a single 

 oar worked at the stern. 



Why and exactly how the cilium of the epithelial cells bends 

 swiftly and straightens slowly, always acting in the same direction, is 

 a problem difficult at present to answer fully. Some have thought 

 that the body of the cell is contractile, or contains contractile 

 mechanisms pulling upon the cilia, which are thus simple passive 

 puppets in the hands of the cells. But there is no satisfactory 

 evidence for such a view. On the whole the evidence is in favour 

 of the view that the action is carried out by the cilium itself, that 

 the bending is a contraction of the cilium, and that the straight- 

 ening corresponds to the relaxation of a muscular fibre. But 

 even then the exact manner in which the contraction bends and 

 the relaxation straightens the filament is not fully explained. 

 We have no positive evidence that a longitudinal half, the inside 

 we might say, of the filament is contractile, and the other half, the 

 outside, elastic, a supposition which has been made to explain the 

 bending and straightening. In fact no adequate explanation of 

 the matter has as yet been given, and it is really only on general 

 grounds we conclude that the action is an effect of contractility. 



In the vertebrate animal, cilia are, as far as we know, wholly 

 independent of the nervous system, and their movement is pro- 

 bably ceaseless. In such animals however as Infusoria, Hydrozoa, 

 &c. the movements in a ciliary tract may often be seen to stop and 

 to go on again, to be now fast now slow, according to the needs 

 of the economy, and, as it almost seems, according to the will 

 of the creature ; indeed in some of these animals the ciliary move- 

 ments are clearly under the influence of the nervous system. 



Observations with galvanic currents, constant and interrupted, 

 have not led to any satisfactory results, and, as far as we know at 

 present, ciliary action is most affected by changes of temperature 

 and chemical media. Moderate heat quickens the movements, but 

 a rise of temperature beyond a certain limit (about 40"C. in the case 



