THE CONTRACTILE TISSUES. 



173 



Ciliary Movements. — This subject has been already con- 

 si4ered briefly in connection with some of the lower forms of 

 life presented for study. 



It is to be noted that there is a gradual replacement of this 

 form of action by that of muscle as we' ascend the animal 

 scale ; it is, however, retained even in the highest animals in 

 the discharge of functions analogous to those it fulfills in the 

 invertebrates. 



Thus, in Vorticella, we saw that the ciliary movements of 

 the peristome caused currents that carried in all sorts of parti- 

 cles, including food. In a creature so high in the scale as the 

 frog we find the alimentary tract ciliated ; and in man himself 

 a portion of the respiratory tract is provided with ciliated cells 

 concerned with assisting gaseous interchange, a matter of the 

 highest importance to the well-being of the mammal. As be- 

 fore indicated, ciliated cells are found in the female generative 

 organs, where they play a part already explained. 



It is a matter of no little significance from an evolutionary 

 point of view, that cil- 

 iated cells arfe more 

 widely distributed in 

 the foetus than in the 

 fully developed ani- 

 mal. 



As would be ex- 

 pected the movements 

 of cilia are afi^ected by 

 a, variety of circum- 

 stances and reagents ; 

 thus, they are quick- 

 ened by bile, acids, 

 alkalies, alcohol, ele- 

 vation of temperature 

 up to about 40° C, 

 etc. ; retarded by cold, 

 carbonic anhydride, Fio. 16' 

 ether, chloroform, etc. 



In some cases their 

 action may be arrested 

 and re-established by 

 treatment with rea- 

 gents, or it may recommence without such assistance. 



Nodes of Eanvier and lines of Fromanu 

 (Kanvier). A. Intercostal nerve of the mouse, 

 treated with silver nitrate. B. Nerve-fiber from 

 the sciatic nerve of a full-grown rabbit. j4, node 

 of Ranvicr ; jj/, medullary substance rendtjred 



F, axis- 



which 

 ly distinct near the node. The lines are I ess 

 marked at a distance from the node. 



All this 



