THE SENSE OF EQUILIBRIUM. 407 



the nerves Supplying the ampullae usually gives rise to marked forced move- 

 ments, as shown in somersaults, spiral swimming, etc., when set free in the 

 water. When, however, the nerves are cut with great eare, with sharp scis- 

 sors, so as to avoid traction on or crushing of the nerves, such forced move- 

 ments do not follow. 



Lee 1 found that when a fish is turned in different positions there is a 

 compensatory change in the direction of the fins and the optic axes determined 

 by the semicircular canal in whose plane the movement is made. He con- 

 cludes that "Each canal has a principal and a subordinate function. The 

 former is the appreciation of rotational body movements in its own plane and 

 toward its side of the body ; the latter is the appreciation of similar move- 

 ments, but in the opposite direction." Electric stimulation of the ampullary 

 nerves or mechanical pressure upon the ampulla? excites equally definite 

 movements of eyes and fins, and the ocular result of nerve-irritation is the 

 exact opposite of that of nerve-section. 



The difference in function between the divisions of the internal ear is 

 indicated by investigations on albinos. White animals with blue eyes are 

 deaf, but possess the normal power of equilibration. Rawitz 2 found the 

 cochlea in such creatures to be much reduced and the organ of Corti atro- 

 phied, while the semicircular canals were normal. 



According to Lee 3 and others, the equilibrium of rest and motion, or static 

 and dynamic equilibrium, depends upon the irritation of different nerve-ter- 

 minals. The manner of action of the latter has been considered. As to the 

 nervous mechanism on which static equilibrium depends, Lee is of the opinion 

 that the knowledge of the position of the head while at rest comes from the rela- 

 tion of the otoliths in the vestibular sacs to the nerve-endings on the maoulce 

 acustlcce. These otoliths form considerable masses in the ears of fishes, and the 

 intensity and direction of their pressure upon hair-cells must vary with the 

 spatial relations of the head, and thus be comparable, in the sense of posi- 

 tion which they arouse, to the tactile sensations derived from the soles of 

 the feet in man. 



The opinion may be ventured that in the semicircular canals we have a 

 sense-organ of a peculiar kind. The evidence is satisfactory that impulses 

 generated in the nerves of the ampulla?, and probably of the vestibular sacs 

 also, give rise to sensations of position both dynamic and static And it is 

 highly probable that such sensations form a constant basis for our notion of 

 the spatial relations of the head. lint the preservation of equilibrium does 

 not depend wholly upon the special sense-organ, as does sight upon the eye. 

 For the muscular and tactile, not to speak of the visual and other senses, 

 supply information in the same direction, and, no doubt, these may to a cer- 

 tain extent vicariously fill the function of the semicircular apparatus when 

 this is abolished. 



1 Lee: Journal of Physiology, xv. p. :;il ; xvi. p. 192. 



2 Rawitz : Zoologiseher Jahresbericht, 1896. 



8 Journal of Physiology, xv. p. 31 1, xvi. p. 192. 



