Introduction to Animal Morphology. 13 



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 epithelium, 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 chitinous 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 skin, 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 cells overlap each other 

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



J Found in glands, and in the deep new layers of stratified epithelium as 

 a transitional form. Found in the digestive tract. 



II Ciliated columnar epithelium occurs in the trachea and bronchi, the 

 Eustachian and Fallopian tubes, &c. Ciliated .spheroidal, often flatlcm-d, 

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

 These cells send dec; from their attached sides. Ciliated tessel- 



lated epithelial cells occur in the tympanic cavity of Mammals at the hinder 

 edpe of the mrmbrana tympnni, and directly behind the attachment of that 

 mcmbran 



