The Adult Tissues of the Human Body 25 



have cilia projecting from the free surface, as in the lining of the bron- 

 chial tubes, the lining of the efferent ducts of the testes, and certain 

 cells of the inner ear. Certain other cells are active secreting organs, such 

 as the ' goblet cells ' which secrete mucous. 



The various epithelial tissues of the adult body develop from all three 

 embryonic layers. 



2. Connective tissue. This is derived from the mesenchymal cells of 

 the mesoderm, and is typically composed of cells with intercellular spaces 

 largely filled with substances secreted by the cells, notably fibers of two 

 sorts, white, and elastic. In some connective tissues the bundles of fibers 

 are closely packed and are generally parallel ; in others (areolar e and 

 reticular connective tissues), the fibers are more loosely arranged, and 

 run in various directions [Fig. 12]. 



m 



m 





Fig. 13. Mucous connective tissues from umbilical cord (navel string) of eight-inch 

 foetal pig. Magnified 600 diameters. (Bailey, Histology.) Fibers have begun to 

 form in the ' ground substance ' between the cells. 



Some connective tissue has no well-developed fibers, the intercellular 

 spaces being filled with gelatinous substance. This is mucous tissue 

 [Fig. 13], and in it the cells show plainly the typical mesenchymal form. 



8 Areolar means literally having spaces between the parts: hence loosely arranged. 

 This word must be distinguished from alveolar, which means literally having cavities 

 or cells (as honey-comb, for example). Reticular means having the form of a net, or 

 network. 



