FUNCTIONAL IRRITABILITY. 329 



cle consists of a series of minute fibrils, which in their 

 turn contain little granules at certain intervals corres- 

 ponding to these discs, we conclude therefrom with some 

 degree of assurance that really local changes take place 

 in the minutest elements, though they cannot be further 

 referred to any visible or directly recognizable cause. 

 We cannot perceive any definite chemical change, or 

 any alteration in the state of nutrition of the parts ; 

 we only see a displacement, a dislocation of the parti- 

 cles, which, however, probably depends upon some 

 slight chemical change in the molecules composing them. 



In the case of ciliated epithelium you see how the 

 fine cilia, which are seated upon the surface of the cells, 

 move in a certain direction, and in this direction exercise 

 a locomotory effect upon the little particles which come 

 near to them. If we isolate the individual cells, we 

 see that every one of them has at its upper end a bor- 

 der of a certain thickness, from which little hair- 

 shaped prolongations run out. These all move in such 

 a way that a cilium which, whilst quiet, stands quite 

 upright, bends forwards and then throws itself back- 

 wards. But we are unable to perceive any changes 

 within the individual cilia, by means of which the move- 

 ment is effected. 



Just the same is the case with gland-cells, con- 

 cerning which we cannot entertain the least doubt that 

 they produce a definite locomotory effect. For since 

 Ludwig has shown in his researches on the salivary 

 glands, that the pressure of the outward current of 

 saliva is greater than that of the inward stream of blood, 

 the only conclusion that is left us is, that the gland-cells 

 exercise a definite motor influence upon the fluid ; and 

 that the secretion is driven out with a definite force, 

 which is not due to the pressure of the blood, or any 

 special muscular action, but to the specific energy of the* 



