FIG. 24. 



THE EPITHELIAL TISSUES. 3! 



many hours, and even for days ; the cells of cold-blooded animals in 

 general continue to vibrate longer than those of mammals. 



The rapidity of the ciliary motion is readily influenced by tem- 

 perature and reagents. While the application of gentle heat stimu- 

 lates, the motion is temporarily arrested by a reduction to 5 C, and 

 permanently impaired by an elevation above 50 C. Increased 

 motion is at first produced by the addition of weak alkalies or acids, 

 followed, however, by a permanent suspension after the prolonged 

 action of these reagents. Cold, chloroform, etc., on the contrary, 

 effect a prompt reduction and, finally, stoppage of the vibrations. 



On surfaces clothed with columnar epithelium, certain cells are 

 distinguished by unusually clear protoplasm and exceptional size; 

 these are the goblet- cells, whose peculiar elliptical or chalice form 

 results from the accumulation of mucoid substance 

 elaborated within their protoplasm. When the dis- 

 tention becomes too great, the cell bursts in the 

 direction of least resistance, evidently towards the 

 free surface, and the secretion is poured out on the 

 surface of the mucous membrane. Goblet-cells 

 occur on all surfaces covered by columnar epithelium, 

 but with especial profusion in the large intestine. 

 These elements may be regarded as corresponding 

 to the unicellular glands of the lower animals ; in 

 the large mucous glands, as the mucous acini of the 

 submaxillary and sublingual, the majority of the secreting elements 

 are in a condition similar to that of the goblet-cells. 



The protoplasm of epithelial cells often becomes invaded by par- 

 ticles of foreign substances ; thus, granules of fatty and proteid 

 matters are very commonly encountered, while 

 the presence of granules of eleidin in certain 

 cells of the epidermis characterizes the stratum 

 granulosum. When these invading particles are 

 colored, as when composed of melanin, the pro- 

 toplasm of the affected cell acquires a brown or 

 black tint, and is then known as pigmented 

 epithelium ; such cells are constant in the deeper 

 layers of the epidermis, especially of certain races, 

 and in the outer layer of the retina. 



Specialized Epithelium. Reference has 

 been made to the goblet-cells as being, tempo- 

 rarily at least, sufficiently specialized to represent 

 unicellular glands ; when the elements become permanently modified 

 to engage in the elaboration of secretion they are recognized as 

 glandular epithelium. 



Goblet - cells from 

 large intestine con- 

 taining mucous secre- 

 tion. 



FIG. 25. 



Pigmented epithelium 

 from outer layer of ret- 

 ina : the nuclei () still 

 uninvaded. 



