introduction to Animal Morphology. i^ 



CHAPTER III. 



HISTOLOGY. 



The tissues formed by the grouping and differentiation 

 of cells may be classified as vegetative or passive tis- 

 sues (connective and epithelial), and animal or active 

 tissues (nerve and muscle). The action of the same 

 forces which in a simple cell cause the formation of a 

 cell-wall, in a mass of cells causes differentiation of the 

 surface plastides into an epitheliicm, whose component 

 cells have more or less thick, keratine walls. When ad- 

 herent together so as to form a porous lamina, this is called 

 cuticle, and in it the cell elements may not be recog- 

 nizable.* Cells on a surface exposed to vertical pres- 

 sure, or to the drying influences of the air, are scale- 

 likef (pavement epithelium) ; when newly formed and 

 unmodified, they are spheroidal ;:|: if laterally com- 

 pressed, moist, and on a free surface, they are columnar 

 or conical,§ and this form often exhibits a covering of 

 cilia.ll Under special circumstances, epithelial cells 



* The cHtinous cuticle of Arthropods can scarcely be recognized as of 

 connected cells, but as far as its development is known it seems to be a 

 chitinization of a thin protoplasm layer. 



t As on the sldn, lips, tongue. It may be unilaminar (Monoderic) or 

 stratified (Polyderic). Sometimes its cells are spinose, ribbed, or toothed, 

 as on the amnion'of the cat (Eberth). When the ceUs overlap each other 

 they are squamose ; when they touch only at their edges they are tesselated. 



X Found in glands, and in the deep new layers of stratified epitlielium as 

 a transitional form. \ Found in the digestive tract. 



II CUiated columnar epithehum occurs in the trachea and bronchi, the 

 Eustachian and Fallopian tubes, &c. Cihated spheroidal, often flattened 

 cells occur in the ependyma on the choroid plexus in the human brain. 

 These cells send deep processes from their attached sides. Ciliated tessel- 

 lated epitheUal cells occur in the tympanic cavity of Mammals at the hiader 

 edge of the membrana tj-mpani, and directly behind the attachment of that 

 membran 



