NATURE OF CILIARY MOVEMENT. 425 



termed cilia has the most important connection with the vital 

 functions, it seems desirable to introduce here a more particular 

 notice of them. They are always found in connection with cells, 

 of whose substance, as we have seen among the Protophyta 

 ( 154, 158), they may be considered as extensions. The form 

 of the filaments is usually a little flattened, and tapering gradu- 

 ally from the base to the point. Their size is extremely varia- 

 ble ; the largest that have been observed being about l-500th of 

 an inch in length, and the smallest about l-13,000th. When in 

 motion, each filament appears to bend from its root to its point, 

 returning again to its original state, like the stalks of wheat when 

 depressed by the wind ; and when a number are affected in suc- 

 cession with this motion, the appearance of progressive waves 

 following one another is produced, as when a wheatfield is agi- 

 tated by successive gusts. When the ciliary action is in full 

 activity, however, little can be distinguished save the wiiirl of 

 particles in the surrounding fluid; but the back-stroke may often 

 be perceived, when the forward-stroke is made too quickly to be 

 seen ; and the real direction of the movement is then opposite 

 to the apparent. In this back-stroke, when made slowly enough, 

 a sort of " feathering" action may be observed ; the thin edge 

 being made to cleave the liquid, which has been struck by the 

 broad surface in the opposite direction. It is only when the rate 

 of movement has considerably slackened, that the shape and 

 size of the cilia, and the manner in which their stroke is made, 

 can be clearly seen. It has been maintained by some, that the 

 action of the cilia is muscular ; but they are often too small to 

 contain even the minutest fibrillse of true muscular tissue, and 

 no such elements can be discerned around their base ; their pre- 

 sence in Plants, moreover, seems distinctly to negative such an 

 idea. Hence we must consider them as organs sui generis, 

 wherein the contractility of the cell to which they belong, is (as 

 it were) concentrated. We have seen that in the Rhizopods, the 

 entire mass of whose sarcode is highly contractile, no cilia are 

 present; whilst in. the Infusoria, whose bodies have compara- 

 tively little contractility, the movements are delegated to the 

 cilia. Cilia are not confined, however, to Animalcules and 

 Zoophytes, but exist on some of the free internal surfaces, espe- 

 cially the walls of the respiratory passages, of all the higher 

 animals, not excepting Man himself. Our own experience 

 assures us that their action takes place, not only without any 

 exercise of will on our own parts, but even without affecting our 

 consciousness; and it has been found to continue for many hours, 

 or even days, after the death of the body at large. How far it is 

 subject to any conscious control on the part of these Animalcules, 

 in which the cilia serve as instruments for locomotion as well as 

 for bringing to them food or oxygen, it is impossible for any one 

 to say with confidence. In this important respect, however, the 

 ciliary movement of Animalcules differs from that which is 



