246 THE TISSUES. 



the alimentary canal, nearly to the anus, is both secretive (of mucus) 

 and protective. The cells covering the villi are also believed to be 

 subservient to absorption of alimentary materials into the blood. 

 Indeed, epithelium is everywhere remarkably endosmotic. In the 

 cervix uteri it is more especially for secretion; in the excretory 

 ducts of all glands, secretive and protective; as it is also in the male 

 urethra and all ducts opening into it, and in the vas deferens. 



Ciliated epithelium (whether simple or compound conoidal) owes 

 its peculiarities to its cilia. Independently of them, it may be, and 

 probably always is, secretive and protective. It lines only the whole 

 of the air-passages (except the air-cells), and the passages opening 

 into them, and a part of the genital passages of the female. Its 

 peculiar function, in the former case, seems to be to secure the con- 

 tact of new portions of air in the air-passages, air-cells, and others, 

 to subserve the function of aeration. The cilia may also aid in 

 preserving a due state of moisture on every part of a membrane, 

 or to prevent occlusion of narrow passages by a normal or abnor- 

 mal secretion as in the Eustachian tube, the lachrymal duct and 

 sac, the finest bronchial tubes, and the Fallopian tubes. 



It has been suggested that the cilia on the cells covering the 

 upper two-thirds of the uterine cavity, and lining the Fallopian 

 tube, carry the semen to the ovary to secure impregnation; and that, 

 by a reversed action, they also return the impregnated ovum to the 

 uterine cavity, where it remains to be developed during the period 

 of gestation. Though this idea of reversed action is purely hypo- 

 thetical, it is still probable that the cilia have reference to the func- 

 tion of menstruation or impregnation, or both, since they are not 

 developed till the period of puberty arrives. But it is a gratuitous 

 assumption that the cilia of the cavities in the face (antrum, &c.) are 

 subservient to smell, since we know that the olfactory nerves are 

 not distributed to these cavities at all. 



Since secretion is in all cases performed by epithelial cells, all the 

 normal secretions contain them, or at least their nuclei or their 

 debris; as has already been seen in the description of them respect- 

 ively in the Second Division of this work. 



Epithelium is corrugated and rendered opaque by the action of 

 alcohol, and hence the effect of holding brandy, &c., in the mouth. 

 In some diseases it becomes entirely detached ; and thus is produced 

 the extreme redness of the tongue which is so often met with. An 

 irritable condition of the mucous membrane of course results from 



