THE SEMICIRCULAR CANALS 629 



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 this 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 

 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 circular 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 coordination, 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 coordinate 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 

 animal shows instability of position in that plane. It has been shown that 

 stimulation of a sectioned canal produced reflex movements in that plane. 



Muscular coordination is a complex phenomenon and involves operation 

 of numerous sensory impulses from other organs of the body, especially from 

 the eye and general skin. Some confusion has arisen from the fact that there 

 are associated with the disturbance in the semicircular canals movements 

 of the eyes and head in higher animals, and of the eyes, head, and fins in such 

 animals as fishes the so-called compensatory movements. Without going 

 into details, it is sufficient to state that the sense organs of the semicircular 

 canals probably form only one of the series of sensory structures concerned 

 in the coordination of movements. 



The Utriculus and Sacculus. The utriculus and sacculus each have 

 a sensory area, the maculae, over which there rests in the human ear 



