i8 



ELEMENTS OF HISTOLOGY. [Chap. HI. 



part facing a cavity, canal, or general surface in many 

 instances a bright thinner or thicker cuticular struc- 

 ture is seen, with more or less distinct vertical striation. 

 The conical or spindle-shaped, club-shaped, and pear- 

 shaped cells are drawn out into longer or shorter 

 single or branched extremities. 



The squamous or pavement cells are cubical, 

 polyhedral or scaly. The nucleus of the former is 

 almost spherical that of the latter flattened in propor- 

 tion to the thinness of the scales. In polyhedral 

 cells it can be occasionally perceived that the granula- 

 tion is due to the regular honeycombed nature of the 

 cell protoplasm. 



19. As regards size, the epithelial cells differ 

 considerably from one another in different parts, and 

 even in the same part. Thus, the columnar cells, 



covering the surface of 

 the villi of the small in- 

 testine, are considerably 

 longer than those lining 

 the mucous membrane of 

 the uterus ; the columnar 

 cells lining the larger 

 ducts of the kidney are 

 considerably longer than 

 those lining the small 

 ducts \ the polyhedral 

 cells -covering the anterior 



surface of the cornea are considerably smaller than 

 those on the surface of the lining membrane of the 

 urinary bladder ; the scales lining the ultimate recesses 

 of the bronchial tubes the air cells are considerably 

 smaller than those on the surface of the membrane 

 lining the human oral cavity and oesophagus. 



20. As regards arrangement, the epithelial cells 

 are either arranged as a single layer or are stratified, 

 forming several superimposed layers; in the former case 



Fig. 13B. Three Mucus-secreting 

 Goblet Cells. 



A, From the stomach of newt ; B, from 

 a raucous gland ; c, from the surface 

 of the mucous membrane of the in 

 testiue. 



