34 HANDBOOK OF PHYSIOLOGY. 



contain fat or other substances, of which the most likely is mucin or 

 its antecedent mucigen, to be seen in the form of granules. It is to the 

 presence of mucin that a curious transformation which columnar cells 

 may undergo is due, and from which the alteration in their shape 

 whereby "goblet-cells " are produced (fig. 27) arises. These altered cells 

 are hardly ever evident in a perfectly fresh specimen; but if such a 

 specimen be watched for some time, little knobs are seen gradually to 

 appear on the free surface of the epithelium and are finally detached; 

 these consist of the cell-contents which are discharged by the open 

 mouth of the goblet, leaving the nucleus surrounded by the remains of 

 the protoplasm in its narrow stem. 



This transformation is a normal process which is continually going 

 on during life, the discharged cell-contents contributing to form mucus, 



Fig. 28. Cross section of a villus of the intestine, e. Columnar epithelium with striated 

 border; g. goblet cell, with its mucus partly extruded; Z, lymph -corpuscles between the epi- 

 thelial cells; b, basement membrane; c, sections of blood capillaries; w, section of plain muscle 

 fibres : c. J, central lacteal. (Schafer.) 



the cells themselves being supposed in many cases after discharge to 

 recover their original shape. 



Ciliated epithelium consists of cells which are generally cylindrical 

 in form (figs. 29, 30), but may be spheroidal or even almost squamous. 



This form of epithelium lines (a.) the mucous membrane of the 

 respiratory tract beginning just beyond the nasal aperture and com- 

 pletely covering the nasal passages, except the upper part to which the 

 olfactory nerve is distributed, and also the sinuses and ducts in connec- 

 tion with it and the lachrymal sac; the upper surface of the soft palate 

 and the naso-pharynx, the Eustachian tube and tympanum, the larynx, 

 except over the vocal cords, to the finest subdivisions of the bronchi. 

 In part of this tract, however, the epithelium is in several layers, of 

 which only the most superficial is ciliated, so that it should more accu- 

 rately be termed transitional (p. 37) or stratified, (b.) Some portions 

 of the generative apparatus in the male, viz., lining the " vasa efferentia " 

 of the testicle, and their prolongations as far as the lower end of the 



