201 CILIARY MOTION. 



There is no reason to believe that ciliated epithelium-cells are in connection either with 

 nerve-fibres, or with the cells of the subjacent connective tissue. An anatomical connection 

 with subjacent cells and fibres has been described in reference to the columnar ciliated 

 epithelium of the central canal of the spinal cord and of the Sylvian aqueduct. But this is 

 a most difficult point to determine exactly, and even if such a connection should be proved, 

 the cells in the situations above mentioned are entirely different in many respects from 

 ordinary ciliated cells. They are relatively slender, and their fixed non-ciliated ends pass into 

 branching fibres, which lose themselves in a network which underlies the epithelium, and 

 appears to be formed chiefly, if not entirely, by the interlacement of the ramified cell- 

 processes. These peculiar ciliated cells closely resemble those which constitute the structures 

 known as nerve-epithelia in some of the lower invertebrata, and which in some of these 

 animals represent the whole central nervous system. 



The cilia themselves differ widely in size in different animals, nor are they of 

 equal size in all parts of the same animal. In the human windpipe they measure 

 _ Q i oo th to a-sVoth of an inch in length ; but in many invertebrate animals they are 

 much larger than this, and in the human epididymis are from eight to ten times 

 longer than in the trachea. 



In figure they have the aspect of slender conical, or slightly flattened filaments ; 

 broader at the base, and usually pointed or rounded at their free extremity. Their 

 substance is transparent, soft and flexible. It is to all appearance homogeneous, 

 and no fibres, granules, or other indications of definite internal structure, have been 

 satisfactorily demonstrated in it. 



The flagellum of Noctiluca, which bears a general resemblance to a large cilium 

 and has a similar rhythmic lashing action, is transversely striated, and the cilium 

 or tail of a spermatozoon also shows certain indications of structure, but nothing of 

 the kind has been observed in ordinary cilia. 



Nature of ciliary movement and influence of varying conditions and reagents. 

 If the cilia be detached from the cell they cease to move, and on this account it is thought by 

 some, that the movement is entirely a passive one, caused by movements in the cell-protoplasm 

 acting upon the rootlets of the cilia. But the apparently independent motion of the tails of the 

 spermatozoa, which are comparable to long single cilia, and that of the long cilia which are 

 protruded from many of the lower animal and plant organisms, has led other authorities to 

 believe that the movement is due to the contraction of the cilia themselves. 



There is, however, a third mode of explanation of the movements which may be suggested 

 and which would have the advantage of bringing them into close relationship with the amoaboid 

 movements of cell-protoplasm, and, as we shall afterwards see, with the process of contraction 

 and extension of muscle. The explanation is briefly as follows : If we suppose that a cilium 

 is a hollow curved extension of the cell, occupied by hyaloplasm, and invested by a delicate 

 elastic membrane (perhaps an extension of the spongioplasm), then it must follow that if 

 there be a rhythmic flowing of hyaloplasm from the body of the cell, into and out of the 

 cilium, an alternate extension and flexion of that process would thereby be brought about. 

 The same result would be got, supposing the cilium to be a straight and not a curved exten- 

 sion of the cell, if the enveloping membrane were thicker Cor otherwise less extensible) along 

 one side than along the other. This last assumption would also enable one the better to 

 account for the spiral direction of the movement of certain cilia ; for this form of movement 

 would be produced if the line of lessened extensibility in these cilia were to pass in a cork- 

 screw fashion along the cilium in place of straight along one side, as assumed for ordinary cilia. 



The cilia vibrate with a frequency of not less than ten times in a second when moving 

 actively, but the rate of movement may be much slower than this. The movement of cilia is 

 incessant so long as the cells remain alive, but that of spermatozoa often exhibits intervals 

 of rest alternating with periods of rhythmic movement. 



The manner in which cilia move, is best seen when they are not acting very briskly. The 

 motion of an individual cilium may be compared to that of a carter's whip, the cilium being 

 rapidly flexed in one direction, that of the current which they produce, and returning more 

 slowly in the other direction. The motion does not involve the whole ciliated surface at the 

 same moment, but is performed by the cilia in regular succession, giving rise to the appearance 

 of a series of waves travelling along the surface, like the waves caused by the wind in a field 

 of com. When they are in very rapid action the undulation is less obvious, and, as Henle 

 remarks, their motion then conveys the idea of swiftly-running water. The undulating move- 

 ment may be beautifully seen on the gills of a mussel. The undulations, with some exceptions 

 seem always to travel in the same direction on the same parts. The impulsion, also, which 



