10 Animal Life and Intelligence. 



The essential constituent of animal (as indeed also of 

 vegetable) tissues is protoplasm. This is a nearly colour- 

 less, jelly-like substance, composed of carbon, hydrogen, 

 nitrogen, and oxygen, with some sulphur and phosphorus, 

 and often, if not always, some iron ; and it is permeated 

 by water. Protoplasm, together with certain substances, 

 such as bony and horny matter, which it has the power 

 of producing, constitutes the entire structure of simple 

 organisms, and is built up into the organs of the bodies 

 of higher animals. Moreover, in these organs it is not 

 arranged as a continuous mass of substance, but is dis- 

 tributed in minute separate fragments, or corpuscles, only 

 visible under the microscope, called cells. These cells are 

 of very various shapes spherical, discoidal, polyhedral, 

 columnar, cubical, flattened, spindle-shaped, elongated, and 

 stellate. 



A great deal of attention has been devoted of late years 

 to the minute structure of cells, and the great improvements 

 in microscopical powers and appliances have enabled 

 investigators to ascertain a number of exceedingly inte- 

 resting and important facts. The external surface of a cell 

 is sometimes, but not always in the case of animals, bounded 

 by a film or membrane. Within this membrane the sub- 

 stance of the cell is made up of a network of very delicate 

 fibres (the plasmogen), enclosing a more fluid material (the 

 plasm) ; and this network seems to be the essential living 

 substance. In the midst of the cell is a small round or 

 oval body, called the nucleus, which is surrounded by a very 

 delicate membrane, In this nucleus there is also a net- 

 work of delicate plasmogen fibres, enclosing a more fluid 

 plasm material. At certain times the network takes the 

 form of a coiled filament or set of filaments, and these 

 arrange themselves in the form of rosettes and stars. In 

 the meshwork of the net or in the coils of the filament 

 there may be one or more small bodies (nucleoli), which 

 probably have some special significance in the life of the 

 cell. These cells multiply or give birth to new cells by 

 dividing into two, and this process is often accompanied 



