70 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



4. Electrical Phenomena.— Electrical phenomena may occur in 

 animal and vegetable tissues under various conditions. 



Frictional electricity occurs when dry epidermal tissues (hair, outer 

 epidermis) and other bodies of rough surface are rubbed together, as on 

 the skin and clothing. It has no physiological significance. 



Currents produced by chemical differences in tissues may be seen in 

 plants when a point of the exposed interior is connected with a point of 

 the external surface, the internal section being negative to the exterior. 

 Such currents probably only exist when contact by conductors is made 

 between these two surfaces. 



In certain animal and vegetable tissues there appear to be elementary 

 parts, which are actively efficient in developing an electrical current. 

 Among such phenomena belong the electrical phenomena observed in 

 certain plants, as the Dioneea muscipula; in certain animals, as the torpedo 

 and electric eel, and in the currents developed in muscles and nerves of 

 all animals. The latter will receive consideration under the subjects of 

 Nerve and Muscle. 



III. MECHANICAL MOVEMENTS IN CELLS. 



It has been seen that the processes by which cells absorb and give 

 up liquids and gases are reducible to purely physical laws. We have 

 further alluded to the fact that the characteristics of the nutritive proc- 

 esses in animal as distinguished from vegetable cells is the reduction of 

 complex organic compounds in the former to simple, inorganic substances ; 

 while in the vegetable cell, simple, inorganic, elementary compounds are 

 built up into complex organic matter. In vegetable cells force is, there- 

 fore, rendered latent'; in animal cells force is liberated. 



In the animal cell this liberation of energy may take on the form of 

 animal movements from the contractility of protoplasm ; or it may result 

 in the development of heat or of electricity. The consideration of the 

 processes which lead to this liberation of energy will be deferred until 

 after the chemical constituents of cells have been discussed, while heat- 

 formation and the development of electricity will be studied under their 

 appropriate headings in Special Physiology. 



The movements seen in animal and vegetable organisms may be the 

 result of external causes, such as friction, heat, or chemical action, or 

 they may be apparently spontaneous. 



Two classes of movement may be distinguished : — 



1. Those which are produced by varying tension in the cell-mem- 

 brane, from varying degrees of imbibition of the cell-contents. 



2. Those which are peculiarly protoplasmic in nature. 



1. Motion Produced by Imbibiton in Cells. — The first of these is 

 especially illustrated by many of the forms of motion which occur in the 



