THE TISSUES. 



We give the following classification : 



1. Simple epithelia (with or without cilia). 



(a) Squamous epithelium. 



(^) Cubic epithelium. 



(<:) Columnar epithelium. 



(d] Pseudostratified columnar epithelium. 



2. Stratified epithelia (with or without cilia). 



(a) Stratified squamous epithelium, with superficial 



flattened cells (without cilia). 

 () Transitional epithelium. 

 (<r) Stratified columnar epithelium, with superficial 



columnar cells (with or without cilia). 



3. Glandular epithelium. 



4. Neuro-epithelium. 



I. SIMPLE EPITHELIUM. 



In simple epithelia the cells lie in a single continuous layer. 

 Simple epithelia are very widely distributed. They line almost 

 the entire alimentary tract, the smaller respiratory passages and air 



Fig. 38. Isolated cells of squamous epithe- 

 lium (surface cells of the stratified squamous 

 epithelium lining the mouth): a, a, Cells present- 

 ing under surface ; b, cell with two nuclei. 



Fig. 39. Surface view of 

 squamous epithelium from skin of a 

 frog; X400. 



sacs, trite majority of the gland ducts, the oviducts and uterus, and the 

 central canal of the spinal cord and ventricles of the brain. 



(a) Simple Squamous Epithelium. In simple squamous epi- 

 thelium the cells are flattened. Their contiguous surfaces appear 

 regular, forming, when seen from above, a mosaic. The nuclei lie, 

 as a rule, in the middle of the cell, and if the latter be very much 

 flattened, the position of the nucleus is made prominent by a bulg- 

 ing of the cell at this point. It occurs in the alveoli of the lung. 



(b) Simple Cubic Epithelium. Epithelial cells of this type 

 differ from the above only in that they are somewhat higher. They 

 appear as short polygonal prisms. Their outlines are, as a rule, not 

 irregular, but form straight lines. Cubic epithelium occurs in the 



