THE SEMICIRCULAR CANALS 663 



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 \vhich 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, 

 w r hich receives a division of the vestibular branch of the eighth nerve. 



The function of the semicircular canals is believed to be to give rise to 

 sensations by which we determine the motion of the body in space. It was 

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

 axis, then suddenly stops and opens the eyes, surrounding objects seem to 

 be rotating around the same vertical axis. If the head be inclined so that 

 the face is in the horizontal plane and the rotation around the vertical axis 

 be repeated, then, upon suddenly raising the head into the ordinary position 

 and opening the eyes, objects seem to be moving about the head around the 

 norizontal 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 canal to move over it so as to produce pressure in the 

 funnel of the ampulla. That is, it has the same effect as though the endo- 

 lymph moved in the canal. This relative motion bends the hairs of the hair 

 cells of the crista acustica, thus stimulating the hair cells and giving rise to 

 sensory nerve impulses. When the head suddenly stops rotating the situa- 

 tion is just reversed and there will be a second stimulation, but in the opposite 

 direction. When one considers 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. 



This theory is borne out by the effects of operation on the semicir- 

 cular canals. By the observations of Flourens, injury to the semicircular 

 canals causes disturbances in muscular co-ordination, especially in move- 

 ments 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- 

 tioned, the head falls to one or the other side according to the canal, and the 



