56 PRINCIPLES OF PHYSIOLOGY 



in different degrees of development, those on 

 the surface being fully formed. It is to be 

 noted that all free surfaces are covered by 

 such cells, and it follows that matter 

 can neither enter the tissues of the body, 

 nor issue from them into the external world, 

 without passing through an epithelial layer. 

 Such matters do not, however, pass through 

 epithelium as a fluid passes through a filter, 

 but the matter is modified chemically and 

 physically by the epithelial cells. 



26. Other cells become differentiated into 

 tissues that form, as it were, the framework 

 of the body, moulding the shape of organs, and 

 supporting their constituent structures. These 

 are called the connective tissues. This variety 

 of tissue is so abundant as to exist in every 

 organ, while it forms a framework for every 

 other tissue. Sometimes it is soft and con- 

 sists of delicate fibres forming networks or 

 membranes or cords (as in sinews), but it may 

 be infiltrated with earthy matter, and form 

 a hard structure of bone, and again it may be 

 hard, without earthy matter, in cartilage or 

 gristle. Lying in it, we always find cells in a 



