Page:Popular Science Monthly Volume 75.djvu/347

Rh Although most glands are not under direct nervous control, some are as completely under this control as the majority of muscles are. The best examples of this condition are the sweat glands and the salivary glands. The fact that when the nerves supplied to the salivary gland are stimulated, secretion may take place at a pressure higher than that of the blood supplied to the gland shows conclusively that the production of saliva is not a simple organic filtration process, but is dependent upon action called forth in the secretory cells by a nervous impulse. This view gains additional support from the fact that in the salivary glands nerve fibers have been found to end in connection with the secretory cells. There is therefore every reason to believe that the salivary glands, and the same may be said of the sweat glands, are organs whose secretions are directly controlled by nerves.

As these several examples show, some glands are completely under the control of nerves and others are not. In my opinion the latter represent the primitive state of this form of effector and the former the condition after such organs have been appropriated by the developing nervous system.

Luminous or phosphorescent organs afford another class of effectors which have probably originated independently and fallen secondarily under the influence of the nervous system. These organs, however, have been studied so imperfectly that it is at present difficult if not impossible to get satisfactory evidence as to their exact condition. Some animals have been supposed to possess phosphorescent organs when in reality their luminosity was due entirely to reflection; others like certain earthworms were found to be phosphorescent because their slime contained photogenic bacteria. But aside from these spurious cases there is an abundant range of truly phosphorescent animals, examples of which occur from protozoans to vertebrates. One peculiarity in their distribution is that true phosphorescent animals are not found in fresh water; they are either marine or air-inhabiting.

In all cases where animal phosphorescence has been examined with care, it seems to be dependent upon the production of a special substance by the light-producing cells. This substance is not in the nature of a living, structurally organized material like muscle, for it can be crushed into a paste and still show light. Moreover, Bongardt (1903) dried the phosphorescent organ of a common firefly over calcium chloride and then kept it in a sealed tube from July 16, 1901, till August 3, 1902, a period of over a year. After this the tube was opened and the organ wet with distilled water; in twelve minutes it glowed so that it could be seen at a distance of two meters. Evidence of this kind supports the view that the phosphorescent substance is not living but rather formed material, such as a secretion, and resembles in this respect pepsin or trysin.