THE TISSUES 25 



particles. These cells do not merely take up material from the 

 blood and pass it out, but they may profoundly alter it before 

 getting rid of it. 



4. Ciliated Epithelium (fig. 5, p. 22). The cells are usually 

 more or less columnar, and the free border is provided 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 direc- 

 tion, 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 movement, 

 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 expelled 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. Mueoid Tissue. The cells of the mesoblast of the embryo, 

 which at first lie in close apposition with one another, become 

 separated, remaining attached by elongated processes. Between 

 the cells, a clear, transparent substance makes its appearance, 

 forming a soft jelly-like tissue. This tissue is widely distrib- 

 uted in the embryo as a precursor of the connective tissues, 

 and after birth it is still to be seen in the pulp of a developing 

 tooth and in the vitreous humour of the eye (fig. 7). 



2. Fibrous Tissue. As development advances, the cells of 

 mucoid tissue elongate and become spindle-shaped, and are con- 

 tinued at their ends into fibres (fig. 8). These cells are often 

 called fibroUasts. 



The connective tissues are thus clearly distinguished from 



