78 GENERAL ANATOMY OF THE TISSUES. 



tions in animals. One of its kinds, the liorny 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, (i), among appen- 

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

 birds, of Chelonia, of the Siren and Ornitliorhynchus ; the horny teeth 

 of the cyclostome fishes ; of the Ornitliorhynchus, of the gill rays of 

 fishes, and of Batrachian larvae; the whalebone, 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 inver- 

 tebrata ; 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 tis- 

 sues 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. 



[The epidermic tissue is a frequent constituent of pathological for- 

 mations. It is met with in skin and mucous membrane, forming epi- 

 dermic hypertrophies, as corns and warts ; also in the so-called epidermic 

 or epithelial tumors, and as a covering on the surfaces of adhesions. 

 The variety of the epidermic tissue generally observed in these forma- 

 tions is the pavement-epithelium, with cells slightly modified by mutual 

 pressure, but not differing otherwise from normal pavement-epithelium. 

 Cylindrical and ciliated epithelium occur but rarely as abnormal pro- 

 ducts. Chemically, the epithelium of pathological tissues presents the 

 same reactions as that of normal tissues. — DaC] 



Literature. — Purkinje et Valentin, " De phoenomeno generali et fuii- 

 damentali motus vibratorii continui," Vratisl. 1835 (Discovery of ciliary 

 movement in the higher animals); Henle, "Symbols ad anatom. vill. 

 int.," Berol. 1837 ; " On the distribution of the epithelium in the human 

 body," Berlin, 1838 ; and upon the development of mucus and pus, and 

 their relation to the epidermis (first exact description of the different 

 epidermic cells) ; Valentin, art. Ciliary Motion, in Wagner's " Hand- 

 Avorterbuch ;" Jiische, "De telis epithelialibus in specie et de iis vasorum 

 in genere," Dorp. 1847. [Pathologically, the epidermic tissue'is con- 

 sidered by Mayer ; " These sur les tumours epidermiques," Paris, 1846 ; 

 by Bennett, " On Cancerous and Cancroid Growths," Edinb., 1849 ; 

 by Lebert, in his Monograph, " Du Cancer et du Cancroide de la Peau," 

 Paris, 1851 ; by Virchow, Wurzb. Verhandl., 1850, and in the recent 

 works of Paget, "Surgical Pathology," London, 1853, and of Wedl, 

 " Grundzuge der Pathologischen Histologie," Wien, 1854. — DaC] 



§ 22. Cartilage. — Cartilage consists of a solid, but elastic, bluish, milk- 

 white or yellowish substance, which presents two morphological condi- 



