ORIGIN OF THE NERVOUS SYSTEM 343 



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 sali- 

 vary 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 de- 

 pendent 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 im- 

 possible 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 cer- 

 tain earthworms were found to be phosphorescent because their slime 

 contained photogenic bacteria. But aside from these spiirious 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 sub- 

 stance by the light-producing cells. This substance is not in the na- 

 ture 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 Au- 

 gust 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 re- 

 spect pepsin or trysin. 



