10 HANDBOOK OF PHYSIOLOGY. 



1. The various epithelia (p. 19) furnish abundant examples of 

 mechanical abrasion. As it approaches the free surface the cell be- 

 comes more and more flattened and scaly in form and more horny in 

 consistence, till at length it is simply rubbed off. Hence we find epithe- 

 lial cells in the mucus of the mouth, intestine, and genito-urinary 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 pro- 

 ducing oil-globules in the secretion of milk, fatty degeneration of the 

 muscular fibres of the uterus after birth of the foetus, and of the cells of 

 the Graafian follicle giving rise to the te corpus luteum." (See chapter 

 on Generation.) (b.) Pigmentary degeneration from deposit of pig- 

 ment, as in the epithelium of the air- vesicles of the lungs, (c.) Calca- 

 reous degeneration, which is common in the cells of many cartilages. 



Differences between Plants and Animals. 



Having now considered somewhat at length the vital properties of 

 protoplasm, as shown in cells of vegetable as well as animal organisms, 

 we are now in a position to discuss the question of the differences between 

 plants and animals. It might at the outset of our inquiry have seemed 

 an unnecessary thing to recount the very great distinctions which exist 

 between an animal and a vegetable, but, however great these may be 

 between the higher animals and plants, yet in the lowest of them the 

 distinctions are much less obvious. 



(1.) Perhaps the most essential distinction is the power which vege- 

 table protoplasm possesses of being able to build up new albuminous 

 material out of such chemical bodies as ammoniujn salts, carbonic acid 

 gas and water, together with! mineral sulphates and phosphates. By 

 means of their green coloring matter, Morophyl a substance almost 

 exclusively confined to the vegetable kingdom plants are capable of 

 decomposing the carbonic acid gas, which they absorb by their leaves. 

 The result of this chemical action, which occurs only under the influence 

 of light, is, so far as the carbonic acid is concerned, the fixation of 

 carbon in the plant structures and the exhalation of oxygen. The carbon 

 thus obtained becomes combined with the elements of water absorbed 

 by the roots, to form starch. By the re-arrangement of the elements 

 composing this body, with the addition of nitrogen and sulphur derived 

 from nitrates and sulphates of the soil, vegetable protoplasm can con- 

 struct albumen. Animal protoplasm is incapable of thus using such 

 substances and never exhales oxygen as a product of decomposition. It 

 must have ready-formed albuminous food in order to liye. 



The power of living upon albuminous as well as non-albuminous 

 matter is less decisive of an animal nature; inasmuch as fungi and some 

 other parasitic plants derive their nourishment in part from the former 

 source. 



(2.) There is, commonly, a difference in general chemical composition 



