MORPHOLOGY OF THE CELL 41 



Salts. — Substances which, in solution, are capable of conducting an 

 electric current and of being decomposed by the current, are called 

 electrolytes. When dissolved, their molecules dissociate into atoms or 

 groups of atoms called ions, which bear positive or negative electrical 

 charges. In protoplasm, the salts of sodium, potassium, magnesium, 

 calcium, iron, manganese, and others are electrolytes. The nitrates, 

 chlorides, sulphates, phosphates, and carbonates of these metals are very 

 commonly present in living matter. The majority of them are highly 

 soluble in ordinary liquids, and diffuse through the general protoplasm, 

 where they serve important functions in life processes. 



Protoplasm a Colloidal System. — When a substance in solution is 

 divided into particles which are too minute to be visible even with an ordi- 

 nary microscope, but which are still greater than molecules, that substance 

 is said to be in a colloidal state. It is scarcely to be regarded as in solu- 

 tion. The presence of the particles can be detected with the ultra- 

 microscope, by means of diffracted light, against a dark ground. Owing 

 to the size of the particles^ the substance does not diffuse through mem- 

 branes of parchment or collodion. A colloidal substance may change 

 readily and repeatedly from a liquid to a solid or viscous state, and 

 return again to the liquid state. Other properties of colloids, which can- 

 not be listed here, are due to the fact that the surface of contact be- 

 tween the dissolved substance and the liquid solvent is enormously in- 

 creased by the division of the former into minute parts. Protoplasm is 

 a colloidal mixture, and a study of its general physiology shows that 

 many of its most characteristic properties are dependent upon that fact. 



The Terms Cell and Protoplasm. — In describing the cell doctrine and 

 the structure of cells and protoplasm, the composition of animal bodies 

 has been treated in a manner perhaps so inclusive as to cause confusion. 

 Lifeless materials have necessarily been included in this account at 

 various places. Recognition of their non-living condition may lead the 

 student to inquire how many of the features described are really com- 

 ponents of the cell or protoplasm. 



The matrix of bone and that of cartilage, as has already been pointed 

 out, are non-living cell products. So are the fibers of common connective 

 tissue, and the bulk of ligaments and tendons. But these structures are 

 all intercellular. When non-living things are found within the cells, it 

 is not always easy to decide whether or not they should be regarded as 

 parts of the cells. And if they are parts of the cell, are they parts of the 

 protoplasm? Opinion would probably differ in many cases, and the 

 question may in some instances be purely academic. 



Non-living materials that have been taken into a cell, whether by 

 accident or during the course of its ordinary activities, may be excluded 

 at once. Such materials are the undigested shells of organisms taken 

 in as food, or solid objects accidentally ingested wuth the food organisms. 



