CH. LL] 



THE SEMICIRCULAR CANALS 



755 



even be that increased pressure on one side of a crista is accompanied 

 by diminished pressure on the opposite face of the same crista. 



"One canal can be affected by, and transmit the sensation of 

 rotation about one axis in one direction only; and for complete 

 perception of rotation in any direction about any axis, six canals are 

 required in three pairs, each pair being in the same or parallel planes, 

 and their ampullae turned opposite ways. Each pair would thus be 

 sensitive to any rotation about a line at right angles to its plane or 

 planes, the one canal being influenced by rotation in one direction, 

 the other by rotation in the opposite direction." (Crum-Brown.) 



The two horizontal canals are in the same plane ; the posterior 

 vertical of one side is in a plane parallel to that of the superior 

 vertical of the other side (see fig. 464). 



FIG. 464. Diagram of semicircular canals, to show their positions in three planes at right angles to 

 each other. It will be seen that the two horizontal canals (H) lie in the same plane : and that the 

 superior vertical of one side (S) lies in a plane parallel to that of the posterior vertical (P) of the 

 other. (The student will understand that though in the diagram the canals are entirely separated 

 from one another, that they are really connected, as shown in fig. 460.) (After Ewald.) 



When these canals are diseased in man as in Meniere's disease, 

 there are disturbances of equilibrium : a feeling of giddiness, which 

 may lead to the patient's falling down, is associated with nausea and 

 vomiting. In animals similar results are produced by injury, and the 

 subject has been chiefly worked out on birds by Flourens, where the 

 canals are large and readily exposed, and more recently in fishes, by Lee. 



Thus, if the horizontal canal is divided in a pigeon, the head is 

 thrown into a series of oscillations in a horizontal plane, which are 

 increased by section of the corresponding canal of the opposite side. 

 After section of the vertical canals, the forced movements are in a 

 vertical plane, and the animal tends to turn somersaults. 



"When the whole of the canals are destroyed on both sides 

 the disturbances of equilibrium are of the most pronounced character. 

 Goltz describes a pigeon so treated which always kept its head with 

 the occiput touching the breast, the vertex directed downwards, with 

 the right eye looking to the left and the left looking to the right, 

 the head being incessantly swung in a pendulum-like manner. 

 Cyon says it is almost impossible to give an idea of the perpetual 



