HISTOLOGY 



the sheath were equally developed about the entire circumference of the 

 axis, the cilia should be able to strike in any direction. Usually the 

 effective stroke is in one direction only, but in some cases it may be 

 reversed. In reversible cilia, such as occur on the labia of the sea anemone, 

 the effective stroke is either toward the mouth or away from it, according 

 to the chemical composition of the substances in contact with the 

 cilia (Parker, Amer. Journ. of Physiol., 1905, vol. 3, pp. 1-16). 

 In such a case the contractile material is supposed to be gathered in two 

 bands, on opposite sides of each cilium. In the irreversible cilia, such as 

 are found elsewhere in the sea anemone and in man, the contractile 

 material, according to Parker, must be gathered especially on one side 

 of the supporting axis. 



The whip-like processes, or flagella, which form the tails of sperma- 

 tozoa, may be compared with single cilia. Each springs from a body 

 resembling a centrosome, and consists of an axial filament with a sur- 

 rounding sheath, but whether the filament or the sheath contains the 

 contractile substance is still uncertain. 



Non-motile projections, somewhat resembling cilia, are found in the 

 cells of the epididymis (Fig. 41, b). They have no basal bodies, and 

 lack the distinctness of true cilia. Generally they appear in conical 

 clumps, which have been compared to the hairs of a wet paint brush. 

 They may be concerned with the discharge of secretion. Other non- 

 motile processes of epithelial cells are 

 the tapering projections of the sen- 

 sory cells, apparently designed to re- 

 ceive stimuli. The lining of the 

 central cavity of the spinal cord and 

 ventricles of the brain is also pro- 

 vided with short projections, which 

 may be degenerating cilia. It is 

 questionable whether these are 

 motile. 



Lateral surface. The lateral sur- 

 faces of epithelial cells may be in 

 close contact with one another, 

 sometimes without intervening cell 

 walls; or they may be separated by 



a thin layer of intercellular substance, which is generally fluid. Im- 

 mediately beneath the cuticular border of the cells lining the intestine, 

 the intercellular substance takes the form of a more solid bar encir- 

 cling each cell and binding it to those which surround it. The 

 arrangement of these terminal bars is shown in the diagram, Fig. 42, 

 and in the section Fig. 35, b. If the section passes down through the 



IntercellU' 

 lar sub- 

 stance. 



PIG. 42. DIAGRAM OF THE NETWORK OF TERMI- 

 NAL BARS. 

 The two cells on the left are divided lengthwise 



into halves; the two on the right are drawn 



as complete cylinders or prisms. 



