THE TISSUES 27 



mucin-forming cells. During the so-called resting state of 

 the gland, the cells are building up zymogen. When the 

 gland is active, the cells throw off the material they have 

 accumulated, and it. undergoes a change to zymin. 



(C) Excreting Epithelium does not manufacture materials 

 of use in the animal economy, but passes substances out of 

 the body. Such epithelium is seen in the kidneys, sweat 

 glands, sebaceous glands, mammary glands, and perhaps in 

 the liver. The cells are composed of a granular protoplasm, 

 in which the presence of the material to be excreted either 

 in its fully elaborated condition, or in process of preparation, 

 may frequently be demonstrated e.g. fat globules, iron con- 

 taining particles, &c. These cells do not merely take up 

 material from the blood and pass it out, but they may pro- 

 foundly alter it before getting rid of it. 



4. Ciliated Epithelium (Fig. 5, p. 24). The cells are 

 usually more or less columnar, and the free border is pro- 

 vided with a series of hair-like processes, the cilia, which 

 vary in size in different situations. 



In the living state the cilia are in constant rhythmic 

 motion, each cilium being suddenly whipped or bent down 

 in one direction, and then again assuming the erect position. 



All the cilia on a surface work harmoniously in the same 

 direction, and the movement passes from the cilia of one cell 

 to those of the next in regular order, beginning at one end 

 of the surface and passing to the other. 



As a result of this constant harmonious rhythmic move- 

 ment, any matter lying upon the surface is steadily whipped 

 along it; and since the cilia usually work from the inner 

 parts of the body to the outside, this matter is finally ex- 

 pelled from the body. 



The movements of the cilia are dependent on the changes 

 in the protoplasm, and everything which influences the rate 

 of chemical change modifies the rate of ciliary movement, 

 which may thus be taken as an index of the protoplasmic 

 activity. 



II. CONNECTIVE TISSUES 



1. Mucoid Tissue. The cells of the mesoblast of the 

 embryo, which at first lie in close apposition with one 



