40 



PHYSIOLOGY OF MUSCLE AND NERVE 



them. While their number is generally proportional to the size of the 

 cell, it may also happen that a single cell is equipped with only one 

 or several cilia. When especially long and thick, they are known 

 as flagellse. In the protozoa, these ciliated cells usually extend over 

 the entire surface, while in the metazoa they occupy more restricted 

 regions of the body. They are found, for example, (a) upon the ova 

 and embryos of many invertebrates, fish, and amphibia, (6) upon the 

 epidermis and in the digestive tract of the coelenterates, worms, echino- 

 derms, and molluscs, (c) in the respiratory passage of molluscs, 

 amphibia, fish, birds and mammals, and (d) in the urogenital tract 

 of vertebrates. In man, they are in evidence upon the mucous mem- 

 brane of the nose, lacrimal duct and sac, Eustachian tube and tym- 

 panic cavity, upper portion of the pharynx, larynx with the exception 



FIG. 10. CILIATED CELLS. 

 A, from a liver duct of the garden 

 snail ; B, from mucosa of frog. (After 

 M. Haidenhain.) 



FIG. 11. MOVEMENT OF 



A SINGLE ILIUM. 



A t Progressive in direction of arrow; 

 B, Regressive. (After Verworn.) 



of the vocal cords, trachea and bronchi, uterus, Fallopian tube, vagina, 

 central canal of the spinal cord and the cerebral ventricles. During 

 embryonal life ciliated epithelium is also present in the mouth, esoph- 

 agus and stomach 



The phenomenon of ciliary motion is brought about by a peculiar 

 to and fro movement of these projections. 1 Being firmly anchored in 

 the outer portions of the cells, they swing like pendula along parallel 

 planes and thus avoid striking one another. In many cases, however, 

 the planes in whicn they move are not straight, but curved, similating 

 circles, ovals, or even the course of a whip-lash. The latter is espe- 

 cially true of the flagellse with which, for example, sperm cells are beset. 

 Moreover, if our attention is directed to a single row of cilia, it is 

 noted that this movement is progressive in character, beginning with 

 their position of rest. The latter may be determined most easily 

 by rendering them inactive by means of a narcotizing agent. At this 



1 Erhart, Studien iiber Flimmerzellen, Archiv fur Zellforschung, xxxi, 1910. 



