72 



THE AMERICAN MONTHLY 



[April, 



more nucleoli. There is usually to 

 be distinguished both an intracellular 

 and an intranuclear reticulum, of the 

 fibrils composing which some histolo- 

 gists contend the cilia are but pro- 

 longations. -With this view 1 do not 

 agree, but I consider cilia to be 

 specially differentiated products (the 

 movements of which are not effected 

 simply by the contraction of this in- 

 tracellular network) * having their 

 origin in a thin hyaline layer of proto- 

 plasm which, similar to the stratutn 

 bacillosum of intestinal epithelium, 

 forms a sort of cover to the proper 

 protoplasm of the cell. Vibratile 

 cilia are not protuberances of the cell- 

 wall, but if such exist they pass 

 through it. In many cases the juxta- 

 position of ciliated cells is so inti- 

 mate that this cover may be removed 

 as a cuticle, from quite extensive 

 surfaces. 



Ciliated epithelium is classified as : 



1. Ciliated Pavement Epithelium, 

 in which the cells appear as these of 

 simple pavement epithelium, with the 

 free surface thickly set with cilia. 

 This variety is, for example, to be 

 found in the tympanic cavity and in 

 the ventricles of the brain, as well as 

 in the peritoneal cavity of the triton 

 and frog.t 



2. Ciliated Cylindrical or Columnar 

 Epithelium. Either in a single stra- 

 tum or laminated, this is by far the 

 most common variety, and occurs in 

 the positions shown in the diagram of 

 human ciliated epithelia, and in simi- 



*N'or by tlie contractility which is already 

 inherent in the protoplasm. The indications 

 of their identity with protoplasm cannot be 

 seen in the more differentiated forms of cilia, 

 but in some of the lower organisms, cilia are 

 formed for a time and again drawn in and 

 their substance fused with the protoplasm. 

 This would go to prove that they are differen- 

 tiations of, and that their movements are 

 due to the same cause as, the movements of 

 the protoplasm. 



t In these animals small ciliated pavement 

 cells are found scattered singly or in groups 

 among the non-ciliated cells on the peritoneum 

 pavitale. the mesentry and in the latter ani- 

 mal on the peritoneal surface of the Cy tenia 

 niasmi. 



lar positions in other vertebrates be- 

 sides in a few exceptional situations 

 {e.g., kidneys of some of the lower 

 vertebrates). 



Ciliary Motion. — If we place be- 

 neath the microscope a portion of 

 fresh, ciliated epithelium in which 

 the movement is at its height, we will 

 at first be unable to distinguish the 

 movement of single cilia, or of the 

 cilia as a whole, the movement only 

 being made manifest by the rapid 

 current induced in the fluid bathing 

 the membrane, solid particles sus- 

 pended in this fluid, such as blood- 

 corpuscles and the like, being driven 

 over the surface in rapid flight. After 

 a time however, as the vitality lessens, 

 the movements reach a declining or 

 weakening stage, in which it may be 

 closely inspected and analyzed for a 

 considerable length of time. The 

 cilia of all epithelia will be found to 

 execute regular, periodical and ryth 

 mical motions, vibrating to and fro 

 in planes which generally stand per- 

 pendicular to the surface of the cell. 

 The directions of movement in neigh- 

 boring cilia are parallel and constant, 

 being at the same time usually paral- 

 lel to the long axis of the organ in 

 which they are found. 



Every entire oscillation consists of 

 two partial oscillations of unequal 

 duration ; first a gradual contraction 

 or bending backward, followed by a 

 forward sweep of double the rapidity, 

 the difference in the force of this 

 backward and forward motion causing 

 the current to proceed in one direc- 

 tion. The more forcible stroke is so 

 rapid that in healthy action it may 

 not be seen, the contracting or bend- 

 ing alone being visible ; hence, as we 

 have already indicated, the weakening 

 stage is the best for examination. 

 Quekett* noticed in the gill of a mus- 

 sel a slight movement of the cilia on 

 themselves, each ciljum turning on its 

 axis through the space of a quarter 

 circle, with a movement like the 



* Besides Engolmann and Valentin. 



