TISSUES, ORGANS, AND SYSTEMS. 



../ 



Among the epidermic tissues 

 we may also enumerate the 

 crystalline lens and the ena- 

 mel. 1 The former consists of 

 long tubular cells filled with 

 albumen, which, as the study 

 of development teaches, are 

 formed by the metamorphosis 

 of a part of the epidermis. 

 The latter contains prismatic, 

 densely-ossified long fibres, 

 which, in all probability, are also nothing more than excessively 

 elongated epithelial cells of the enamel organ of the era- 

 brvonic tooth sac. 



[The epidermic tissue is found through almost the whole 

 animal kingdom, and as regards its elements, it exhibits no very 

 considerable deviations in animals. One of its kinds, the horny 

 tissue, appears to occur more generally, and to some extent 

 in peculiar forms. To it belong (a), among structures which 

 appertain to the skin : claws, hoofs, horns, spines, plates, and 

 discs, bristles, feathers, and penis-spines, (b), Among appen- 

 dages of the mucous membranes : the horny sheaths of the beaks 

 of birds, of Chelonia, of the Siren and Omithorhynchus ; the 

 horny teeth of the Cyclostome fishes ; of the Omithorhynchus , 

 of the gill rays of fishes, and of Batrachian larvre ; the whale 

 bone, the spines and plates of the tongue of Birds, Mammalia, 

 and some Amphibia ; the spines of the oesophagus of Chelonia ; 

 the jaws of Cephalopoda and other Invertebrata ; the gastric 

 teeth of many mollusks ; the horny plates of the bird's stomach. 

 In all these structures, but often only by the aid of caustic 

 alkalies, horny plates of one kind or another, as in the corneous 

 structures of man, are discoverable. On the other hand, the 

 hard tissues of the Articulata differ not only morphologically 

 but also chemically from them, consisting as they do of a 

 peculiar substance, chitin, and exhibiting no cellular structure.] 



Fig. 18. Ciliated epithelium from the trachea of a man, x 350: a. outermost part 

 of the elastic longitudinal fibres ; b. homogeneous outermost layer of the mucous 

 membranes; c, deepest round cells; d, median long cells; e, outermost ciliated cells. 



1 [Vide infra, note § on the Teeth.] 



