THE WORK OF ORGANS AND CELLS 45 



the working gland becomes distinctly red. Since the secretion 

 of saliva requires water and this can be obtained only from 

 the blood, it is easy to see why an abundant blood supply is 

 essential to activity. Other constituents of the saliva, such 

 as the inorganic salts, likewise come directly from the blood. 



4. The relation of nerves to gland work ; irritability. Nerves 

 pass, as we have seen (p. 29), from the central nervous sys- 

 tem to the salivary glands. These nerves are essentially 

 bundles of nerve fibers which are distributed from the brain 

 and spinal cord to the neighborhood of the gland cells. 

 Such fibers are the means of conveying to the gland an 

 influence, called a nervous impulse, and nervous impulses 

 cause the gland to secrete. It is also a fact that when these 

 nerves are cut or injured in any way, so that the gland is 

 no longer in nervous connection with the brain and spinal 

 cord, saliva is not secreted, even when food is placed in the 

 mouth. Evidently the activity of the gland is normally 

 aroused by nervous impulses from the brain and spinal cord, 

 just as the activity of a receiving instrument in a telegraph 

 office is aroused by the electric current which comes to it 

 over a wire, or as a mine is exploded by the same means. 

 The gland then stands ready for the act of secretion and is 

 thrown into activity by a nervous impulse from the central 

 nervous system. We speak of this action of a nerve upon 

 the organ in which its fibers end as stimulation and that 

 property of an organ in virtue of which it may be aroused 

 by a stimulus as irritability. 



All the working organs of the body (in contradistinction 

 to the supporting organs, p. 35) are in this sense irritable, 

 and most of them receive nerves which set them to work. 

 Irritable tissues may, however, be stimulated by other means 

 than by nervous impulses. Of these means an electric shock 

 is the most familiar; others are the sudden application of 

 heat, the presence of certain substances in the blood, and 

 even a sharp blow. 



