488 Dynamic Theory. 



tails by the waving of which a progressive motion is accomplished ; for 

 example, the Monads and other Flagellata. None of these organisms 

 have any visible nervous system. Even in the Corals, Sea-anemones, 

 Dead-man's-toes, Sea-pens, &c. , in which some of the cilia are developed 

 and expanded into tentacles or waving arms of considerable size, no 

 distinct nervous system appears to be differentiated. As we shall see 

 further on, all muscle tissue is contractile and capable of receiving stim- 

 ulus either -by w&y of or independent of nervous agency. The nerves 

 are differentiated fibres set off by habitual use to the conve} 7 ance of 

 stimuli to parts of the organization remote from the point of habitual 

 stimulation. Before such differentiation, therefore, the muscle for 

 example, a tentacle itself performs the double office, and receiving the 

 stimulus at one extremity or upon one side, after being affected by it, 

 furnishes it conveyance to the other extremity or the other side, which 

 in turn receives its contracting stimulus. 



The most elementary organs of the nature of muscles in the vertebrate, 

 as well as the invertebrate organism, are the vibratile cilia. These proc- 

 esses are found in various internal parts of Birds, Reptiles and Mam- 

 mals. In Man they are found on the mucous membranes lining the 

 nasal passages, the trachea and its branches, the uterus and its outlet, 

 and the ventricles or cavities of the brain. Such a membrane is called 

 ciliated epithelium, and it is constituted like other epithelium, with a sub- 

 mucous vascular la}^er at the bottom supplying nourishment to the layer 

 of young spherical nucleated cells immediately above ; above that a 

 layer of mature nucleated cells squeezed into the shape of so many ver- 

 tical cylinders standing side and side ; on the top of each of which are 

 several waving cilia. The whole calls to mind a colony of monads stand- 

 ing side by side waving their tails. Their vibrations are constant and 



FIG. 230. Vertical Section of Ciliated Epithelium. 

 a. Sub-layer containing blood-vessels. 

 6. Layer of the young newly formed epithelial cells, 

 c. Mature cylindrical cells. 

 d Cilia on the free surface of the epithelium. 

 ( The air passages of the nose and wind-pipe and cavities 

 of the brain are lined with this sort of epithelium.) 



are independent of the nervous system and be- 

 yond the control of the will. The movement is 

 probably due to the conversion of friction or heat 

 into electrical or nervous movement, by which 

 the cilia are contracted alternately on opposite 

 sides. The movement will sometimes* continue 



J? IG. zoU. 



after the epithelium holding the cilia is detached from the body. This 

 automatic waving motion of the cilia has the effect of aiding the move- 

 ment of mucus over the mucous membrane, and generally in the direc- 

 tion of the outlet to the organ in which it is placed. The palate of the 



