CHAPTER XVI T 

 THE CIVIL ENGINEERS OF THE 1?01)V 



CONNKCnVK TISSriO C'KLLS 



" Which (loth neatly cU'flarc how nature Geometrizeth and observetii ordei- 

 in all things/' Sih Thomas Bhowm:. 



We have just seen how increase in the size of an organism necessi- 

 tates increase in its complexity. Groups of simple naked cells 

 held together only by a pellicle resulting from surface adsorption, 

 would, at the best, form unwieldy organisms which would easily 

 be distorted beyond their elastic limit and so destroyed. Some 

 means of binding the component cells together to form a body 

 sufficiently rigid to withstand shock and yet possessing sufficient 

 mobility to seek its prey and avoid its enemies, is a logical outcome 

 of growth in size and in complexity. Moreover, if the animal is 

 to have an efficient muscular system under complete control and 

 able to be employed under all environmental conditions, some 

 absolutely rigid systems of portable levers and fulcrums must be 

 presupposed. 



The Vegetative Tissues are those which support, bind together, 

 and mechanically protect the other tissues of the body. They 

 may be divided into two groups — the epithelial tissues which 

 protect surfaces, and the connective tissues which bind together 

 and support the \'arious organs of the body. 



I. Epithelium. 



The shape of the cells forming a protective layer or series of 

 protective layers depends entirely on the resultant of the forces 

 acting on them. We may take it for granted that here as well 

 as with the single cell the agency of surface energy is obviously 

 of first importance. 



The theory underlying the phenomena associated with the manifestations 

 of surface energy depends on the principle of Le Chatelier. Surface tension 

 is proportional to the area of the surface of contact. It is also proportional 

 to a coefficient which is specific for each pair of substances provided tem- 

 perature is kept constant. It may be profoundly modified by the slightest 

 alteration in one or more of the physical or chemical conditions of one or 

 both of the phases forming the contact surface. 



203 



