100 



STRUCTURE OF THE VERTEBRATES 



thin, and lie against each other in a tile-like manner. Those in 

 the skin lose the power of division. 



2. Cuboidal epithelium, as the name implies, is composed of 

 cells which are about as deep as they are in width. 



3. Columnar cells are long, one of the smaller faces lying in 

 contact with the opening. This type is naturally the thickest of 

 those epithelia which are only one cell deep. 



Any surface may be covered by epithelium which is a single 

 layer in thickness, or by stratified epithelium composed of sev- 

 eral layers of cells. The former is simple and is easily classified 

 by its type of cells. In stratified epithelia it is not uncommon to 

 have a base of columnar cells with several layers of cuboidal 

 cells above them. These tissues vary in thickness and in type 



Surface 

 view 



Cross section 

 Squamous cells 



Cuboidal (section) 



Columnar (section) 

 Fig. 41. Epithelial Tissues. 



Stratified (section) 



of cells composing the strata. Also, epithelial cells may change 

 their shape when covering a distensible membrane. The squa- 

 mous cells of the arteries become thinner under force of blood 

 pressure, and the cuboidal cells of the bladder become squamous 

 w4ien the organ is enlarged. 



B. Connective Tissues. These are mesodermal tissues which 

 support the other tissues. They are the most abundant tissues 

 of the body, ramifying through every gland and muscle. It is 

 customary to divide them into (1) connective tissues proper 

 and (2) cartilage and bone. 



1. Embryonic tissues are of little importance in a study of 

 the adults of any group, except in pathology. The healing proc- 



