28 HANDBOOK OF PHYSIOLOGY. 



possessing great power of resistance to reagents, contrast strikingly 

 with the homogeneous matter from which they are derived. 



Y. Tubules, such as the capillary blood-vessels, which were originally 

 supposed to consist of a structureless membrane, have now been proved 

 to be composed of flat, thin cells, cohering along their edges. 



Decay and Death of Cells. There are two chief ways in which the 

 comparatively brief existence of cells is brought to an end. (1) Mechan- 

 ical abrasion, (2) Chemical transformation. 



1. The various epithelia furnish abundant examples of mechanical 

 abrasion. As it approaches the free surface, the cell becomes more and 

 more flattened and scaly in form and more horny in consistency, till at 

 length it is simply rubbed off as in the epidermis. Hence we find epi- 

 thelial cells in the mucus of the mouth, intestine, and genitourinary 

 tract. 



2. In the case of chemical transformation the cell-contents undergo 

 a degeneration which, though it may be pathological, is very often a 

 normal process. 



Thus we have (a) fatty metamorphosis producing oil-globules in the 

 secretion of milk, fatty degeneration of the muscular fibres of the uterus 

 after the birth of the foetus, and of the cells of the Graafian follicle 

 giving rise to the "corpus luteum/' (b) Pigmentary degeneration from 

 deposit of pigment, e.g. in the epithelium of the air vesicles of the lungs, 

 (c) Calcareous degeneration, which is common in the cells of many 

 cartilages. 



I. The Epithelial Tissues. 



The term epithelium is applied to the cells covering the skin, the 

 mucous and serous membranes, and to those forming a lining to other 

 parts of the body as well as entering into the formation of glands. For 

 example: 



Epithelium clothes (1) the whole exterior surface of the body, form- 

 ing the epidermis with its appendages nails and hairs; becoming con- 

 tinuous at the chief orifices of the body nose, mouth, anus, and urethra 

 with the (2) epithelium which lines the whole length of the (3) respi- 

 ratory, alimentary, and genito-urinary tracts, together with the ducts of 

 their various glands. Epithelium also lines the cavities of (4) the brain 

 and the central canal of the spinal cord, (5) the serous and synovial 

 membranes, and (6) the interior of all blood-vessels and lymphatics. 



Epithelial cells possess an intracellular and an intranuclear network 

 (p. 9 and 10). When combined together to form a tissue, they are held 

 together by a clear, albuminous, cement-substance, scanty in amount. 

 The viscid semi-fluid consistency both of cells and intercellular sub- 

 stance permits such changes of shape and arrangement in the individual 



