70 



HUMAN ANATOMY. 



Fig. 89, 



■^ 



Goblet-cells from epithelium lining large 

 intestine. X 500. 



Modified Epithelium. — The free surface of the epithehum in many locaHties, 

 as in the trachea, the inferior and middle nasal meatuses, and the uterus, is pro- 

 vided with minute, hair-like vibratile processes, or cilia, which are produced by the 

 specialization of the cytoplasm of the free end of the cell. The e.xact relations of 

 the cilia to the cytoplasm are still matters of uncertainty, although the investigations 

 of Engelmann and others on the ciliated epithelium of invertebrates render it prob- 

 able that the hair-like processes attached to the cells of higher animals are also 

 connected with intracellular fibrillae, which appear as delicate striations within the 

 superficial and more highly specialized parts of the cells. In man and the higher 

 mammals ciliated epithelium is limited to the columnar variety. The exact number 

 of individual cilia attached to the free surface of each cell varies, but there are usually 



between one and two dozen such appendages. Their 

 length, likewise, diff^s with locality, those lining 

 the epididymis being about ten times longer than 

 those attached to the tracheal muccms-membrane. 

 When favorable conditions obtain, including a suffi- 

 cient supply of moisture, oxygen, and heat, ciliary 

 motion may continue for many hours and even days. 

 On surfaces clothed with columnar epithelium 

 certain cells are distinguished by unusually clear 

 cytoplasm and exceptional form and size ; these are 

 the goblet-cells, the peculiar elliptical or chalice 

 form of which results from the accumulation of the 

 mucoid secretion elaborated within their protoplasm. When the distention becomes 

 too great the cell ruptures in the direction of least resistance, and the secretion is 

 poured out upon the surface of the mucous membrane as the lubricating mucus. 

 The goblet-cells, therefore, may be regarded as unicellular glands, and represent the 

 simplest phase in the specialization of glandular tissues. 



The protoplasm of epithelial cells often becomes invaded by particles of foreign 

 substances ; thus, granules of fatty and proteid matters are very commonly encoun- 

 tered, while the presence of granules of eleidin in certain cells of the epidermis char- 

 acterizes the stratum granulosum. When the invading particles are colored, as when 

 composed of melanin, the affected cells acquire a dark brown tint, and are then known 

 as pigmented epithelium. Examples of such cells are seen in the 

 retina and in the deeper cells of the epidermis in certain races. Fig. 90. 



Specialized Epithelium. ^Reference has already been 

 made to goblet-cells as representing unicellular glands ; these 

 may be regarded, therefore, as instances of a temporary 

 specialization of epithelium into glandular tissue. When the 

 epithelial elements become permanently modified to engage in 

 the elaboration of secretory substances, they are recognized 

 as glayididar epithelitim. The cells lining the ducts and the 

 ultimate compartments of glands are modified extensions of 

 the epithelial investment of the adjacent mucous membrane. 

 Their form and condition depend upon the degree of speciali- 

 zation, varying from columnar to spherical and polyhedral, on 

 the one hand, and upon the nature and number of the secre- 

 tion particles on the other. The cells lining parts of certain glands, as those clothing 

 the ducts of the salivary glands, or the irregular portion of the uriniferous tubules, 

 exhibit a more or less pronounced striation ; cells presenting this peculiarity are 

 termed rod-epitheliiim. 



The highest, and often exceedingly complex, specializations affecting epithelial 

 tissues are encountered in connection with the neurones supplying the organs of 

 special sense. The epithelium in these localities is differentiated into two groups of 

 elements, — the sustentacular and the perceptive ; to the latter the name of 7ienro- 

 epithelium is applied. Conspicuous examples of such specialization are the rod- and 

 cone-cells of the retina and the hair-cells of Corti's orean in the internal ear. 



h more detailed description of the glandular tissues is given with the digestive 

 tract ; that of the neuro-epithelia with the organs of special sense. 



Pigmented epithelium from 

 'human retina. X 435- 



