828 PHYSIOLOGY. 



CONJUGATE DEVIATION. 



Waller explains this as follows: The two eyes are exactly equal 

 and parallel for different directions of distant vision. Both eyes are 

 turned to the right or to the left, up or down, so that the object 

 fixed gives images on corresponding parts of both retinae. In move- 

 ments directly upward or downward muscles of the same name in 

 each eye are associated in action ; but in lateral movements the asso- 

 ciation is asymmetrical : e.g.., the external rectus of one eye acts with 

 the internal rectus of the other, and the peculiarity of this asso- 

 ciated action seems still more striking when it is remembered that 

 the external rectus is supplied by the sixth nerve, while the internal 

 rectus is supplied by the third. A similar, if less striking, associa- 

 tion of asymmetrical muscles on the two sides occurs in the rotation 

 of the head and neck, which are turned to the right by the right 

 inferior oblique and the left sterno-mastoid muscles, and to the left 

 by the left inferior oblique and the right sterno-mastoid. In look- 

 ing to the right we contract the right external and left internal 

 recti: i.e., impulses pass through, the right sixth nerve and the left 

 third, possibly from the left and from the right side, respectively, of 

 the motor cortex, but more probably from only the left motor cor- 

 tex, in which case we must suppose that certain nerve-fibers cross 

 twice: once between the cortex and bulbar nucleus and a second 

 time between the nucleus and nerve-termination. Unilateral con- 

 vulsions of cortical origin are accompanied by rotation of the head 

 and eyes toward the convulsed side: i.e.., away from the cerebral 

 lesion. Thus a discharging lesion of the right motor cortex causes 

 convulsions of the left side of the body, with rotation of the eyes 

 to the left. This is a "conjugate deviation." A destructive lesion 

 of the right motor cortex causes paralysis of the left side of the 

 body, with rotation of the eyes to the right. The peculiarity in this 

 case is that there is a cessation of action along the left sixth nerve 

 (external rectus) and the right third nerve (internal rectus), the 

 deviation of the eyes to the right being caused by the unbalanced 

 action of the muscles, which rotate the eyes to the right. 



FIFTH PAIR, TRIGEMINUS, OR TRIFACIAL NERVE. 



The fifth pair of nerves, like a spinal nerve, has two roots: an 

 anterior motor one and a posterior sensory one. The neuraxons of 

 the motor nucleus in the pons make up the motor root. The sen- 

 sory arises in the Gasserian ganglion, and, like a posterior-root 





