CAUSES AND PHENOMENA OF MOTION. 13 



face, or the discharge of a Leyden jar, or of a galvanic shock through it. 

 The vapor of chloroform arrests the motion; but it is renewed on the dis- 

 continuance of the application (Lister). The movement ceases in an at- 

 mosphere deprived of oxygen, but is revised on the admission of this gas. 

 Carbonic acid stops the movement. The contact of various substances 

 will stop the motion altogether; but this seems to depend chiefly on 

 destruction of the delicate substance of which the cilia are composed. 



Nature of Ciliary Action. Little or nothing is known with cer- 

 tainty regarding the nature of ciliary action. It is a special manifestation 

 of a similar property to that by which the other motions of animals are 

 effected, namely, by what we term vital contractility (Sharpey). The 

 fact of the more evident movements of the larger animals being effected 

 by a structure apparently different from that of cilia, is no argument 

 against such a supposition. For, if we consider the matter, it will be 

 plain that our prejudices against admitting a relationship to exist between 

 the two structures, muscles and cilia, rests on no definite ground; and 

 for the simple reason, that we know so little of the manner of production 

 of movement in either case. The mere difference of structure is not an 

 argument in point; neither is the presence or absence of nerves. For in 

 the foetus the heart begins to pulsate when it consists of a mass of em- 

 bryonic cells, and long before either muscular or nervous tissue has been 

 differentiated. The movements of both muscles and cilia are manifesta- 

 tions of energy, by certain special structures, which we call respectively 

 muscles and cilia. We know nothing more about the means by which 

 the manifestation is effected by one of these structures than by the other: 

 and the mere fact that one has nerves and the other has not, is no more 

 argument against cilia having what we call a vital power of contraction, 

 than the presence or absence of stripes from voluntary or involuntary 

 muscles respectively, is an argument for or against the contraction of one 

 of them being vital and the other not so. 



As a special subdivision of ciliary action may be mentioned the motion 

 of spermatozoa (Fig. 403), which may be regarded as cells with a single 

 cilium. 



II. AMOEBOID MOTION. 



The remarkable movements observed in colorless blood corpuscles, 

 connective-tissue corpuscles, and many other cells (p. 8, Vol. I.), must be 

 regarded as depending on a kind of contraction of portions of their mss 

 very similar to muscular contraction. 



There is certainly an analogy between the spherical form assumed by a 

 colorless blood-corpuscle on electric stimulation and the condition known 

 as tetanus in muscles. 



