Page:Life in Motion.djvu/210

190 a motor end-plate in muscle, and in malapterurus to the terminations of nerves in the cells of glands. The molecular disturbance transmitted along a nerve causes changes in its end organ, and these are propagated to the surrounding substance. These changes are associated with a change of potential, and the part becomes negative. A wave of negativity passes through the organ, and there may be a result, the nature of which will, depend on the kind of organ in which the change may take place. If it be a muscle, the chief expression of the change is a variation in form or contraction; if it be a gland-cell, the change is the formation, or disintegration, or modification of certain matters of the secretion; and if it be an electrical organ, it is an electrical discharge. In all three, however, similar phenomena occur, but to varying amounts. Thus, call contraction a, electrical phenomena b, and glandular changes c. In a muscle a is large and b and c small; in a gland a may not occur as an active movement at all, although the cell may change slowly in volume; b is also small, but c is large; and in an electrical organ a is no doubt small (if it