7 X 4 THE SENSES 



These structures have each a special modification of the sensory epithe- 

 lium which receives the vestibular branch of the eighth nerve. These 

 epithelial areas are differentiations of the embryonic ear pit, which is derived 

 from the epiblast. In fishes which have well-developed semicircular canals 

 and vestibule this sensory epithelium has a common origin from the em- 

 bryonic anlage which gives rise to the ear, the branchial sense organ, and 

 the lateral line organs, all of which probably have static functions. 



The Semicircular Canals. The semicircular canals are connected 

 with the utriculus, are three in number on each side, and have been already 

 shown to lie in space practically at right angles to one another. Each is 

 filled with endolymph, and each has a special organ, the crista acustica, 

 which receives a division of the vestibular branch of the eighth nerve. 



The function of the 'semicircular canals has long been believed to be to 

 give rise to sensations by which we determine the motions of the body in 

 space. But Maxwell has shown that motion and position are both medi- 

 ated by the combined reactions of the labyrinthine sense organs. It was 

 shown long ago that if one closes his eyes and turns rapidly around the ver- 

 tical axis, then suddenly stops and opens the eyes, surrounding objects 

 seem to be rotating around the same vertical axis. If the face is in the 

 horizontal plane and the rotation around the vertical axis, then, upon 

 suddenly raising the head into the ordinary position and opening the eyes, 

 objects seem to be moving about the head around the horizontal axis. In 

 both these cases the direction of the apparent motion of objects depends 

 upon the actual motion of the body that preceded it and is in the opposite 

 direction. In the first case the rotation is in the plane of the horizontal 

 semicircular canal. It is assumed here that, at the beginning of such a 

 movement, the endolymph, being fluid and inert, tends to remain still for a 

 moment and the effect is to produce an increase in pressure in the funnel 

 of the ampulla. This relative increase of tension on the hair cells of the 

 crista acustica, stimulates the hair cells and gives rise to sensory nerve 

 impulses. When the head suddenly stops rotating the situation is just 

 reversed and stimulation is in the opposite direction. Considering the 

 position of the three semicircular canals, it will be seen that movement of 

 the head in any direction will stimulate one or more of the cristae, giving 

 rise to either simple or complex sensory impulses. Stimuli on the recessus 

 utriculi adds to the effect. 



This theory is borne out by the effects of operation and stimulation of 

 the exposed semicircular canals. Injury to the semicircular canals causes 

 disturbances in muscular co-ordination, especially in movements that take 

 place in the plane of the injured canal. If a horizontal canal in a pigeon 

 be sectioned, the animal supports its head in the vertical position very 

 well, but is unable to co-ordinate its horizontal movements. It tends to 

 produce rotary motions around the vertical axis. If a vertical canal is sec- 



