CHAPTER XVI 

 THE ARMY FOR HOME DEFENCE 



■■ Whatever uncertainty and variety may appear in the world, \vc remark, 

 nevertheless, a certain secret concatenation and rejjiilar order at all times carried 

 on by rrovidence. which causes everything to proceed in its course, and to i'ollo\v 

 the law of its destiny." La Rochefoucauld. 



There are certain cells and certain cell-communities whose 

 function it is to guard the organism from the invasion of its cells 

 by noxious substances and by predatory parasites. 



1. Ciliated Epithelial Cells. In certain parts of the body, where 

 it might be possible for solid matter in a fine state of division to 

 find its way into hollow visci, a pectiliar type of protective mecha- 

 nism is found. The entrance to the cavity is lined by a layer of 

 more or less cohmmar cells on the exposed surface of each of which 

 is a bunch of fine tapering filaments. During life the cilia move 

 in such a way as to prodtice a flow otitwards of the fluid bathing 

 them. Ciliated epithelitmi is found lining the whole extent of the 

 air-passages (except upper part of nares, lower part of pharynx, 

 terminal bronchioles and pulmonary alveoli, q.v.). It occurs also 

 in the uterus and uterine tubes ; in the efferent tubes of the testes. 

 [As we shall learn later, ciliated epithelium is found in other places, 

 e.g., central canal of spinal medulla and ventricles of the braiit 

 where it prov^okes movement of the cerebro-spinal fluid.] 



There are two phases in the movement of any cilium, (a) a rapid 

 stroke in the direction in which the action is to be effective, and 

 (b) a slow return stroke. Further, although all the cilia attached 

 to any one cell or row of cells act synchronously, there is a meta- 

 chronical rhythm about the whole ciliated stirface, i.e., any indivi- 

 dual cell begins its effective stroke slightly later than the cell 

 immediately internal to it and slightly earlier than the cell lying 

 external to it. In this way a progressive wave motion is pro- 

 duced, carrying towards the exterior the extraneous matter, dust, 

 etc., entangled in the mucus deposited on the ciliated stirface from 

 numerous glands. 



In considering the efficiency of this means of protection, one has 

 to take into account the amplitude of the strokes, the frequency 

 of the strokes of the cilia as a whole, and the exactitude of the 

 timing of their concerted rhythm. Human cilia are not very long, 



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