24 GENERAL ANATOMY. 



all the surfaces of the body, the external skin, as well as the in- 

 terior of cavities and canals, is called Epithelium. (Formerly 

 this term comprehended the covering of the mucous membrane 

 only.) 



It consists of a layer of independent nucleated cells, formed from the or- 

 ganic matter, which the highly vascular surface (matrix) lying immediately 

 underneath it, deposits, and which increases layer by layer. 



20. It does not merely serve as a defence to the tunic upon 

 which it exists, but also for the reception and secretion of certain 

 substances. Wounding it gives rise to neither pain nor bleeding. 



According to the Elementary parts, three kinds are distinguished. (Henle.) 



a. Pavement epithelium. Cells and nuclei roundish, oval, or where lying 

 close to one another, multangular, and conjoined like mosaic. This most 

 frequent variety forms thick layers, and is found upon the internal surface of 

 serous membranes, of the vascular system, upon the mucous membrane of 

 the mouth, the nose, or the globe of the eye. Upon the cornea of the eye it 

 soon becomes dull after death, and appears as a white mucus, whence the 

 dull appearance of the dead eye arises. The salivary mucus, also, consists 

 in great part of this kind. 



To this belongs also the Epidermis. See external skin. 



6. Cylinder epithelium. Cells of a cylindrical or conical shape, the point 

 of which is directed towards the mucous membrane, arranged like fibres. 

 Nucleus generally between the base and apex of the cone. 



They are present in man upon the mucous membranes only. In the ali- 

 mentary canal they commence at the mouth of the stomach (Carefo'a), and ter- 

 minate at the anus ; being most distinct in the cylindrical glands of the large 

 intestines. Upon the mucous membrane of the male organs of generation ; 

 at the opening of the salivary glands. In the gall-bladder they appear 

 coloured green. 



c. Ciliated epithelium is distinguished from the cylindrical simply in the 

 fact of its possessing cilise at one of its extremities. The ciliae themselves 

 are variously formed. Longitudinal diam. of cell= O015 of a line, the ciliae 

 = 0-0018 of a line. 



They exist in man upon the mucous membrane of the respiratory organs, 

 the eyelids, the female organs of generation, and upon the parietes of the 

 cerebral ventricles. It is obtained most readily from the nasal mucous mem- 

 brane. 



Ciliary motion. According to Purkinje and Valentin, the discoverers, it is 

 of three kinds : 



1. The ciliae more in the shape of an inverted cone, since their apices de- 

 scribe a wider circle man their bases. 



2. The whole cilium moves in an undulating manner. 



3. The apex curves like a hook. 



A low temperature (-}- 5 R.), acetic, strong mineral acids and bile arrest 

 the ciliary motion. The stream of the fluid in which the ciliae move is op- 

 j>osed to the direction in which they curve, and generally passes from within 

 to without. 



The object of the ciliary movements is not yet ascertained. In diseases, 

 for example, catarrh, the ciliary epithelium is at first thrown off. 



