544 THE NERVOUS SYSTEM. 



being supplied by the sixth, and the other by the third 

 nerve. So with the oblique muscles : the simplest and 

 easiest actions would be through branches of the corre- 

 sponding nerves, acting similarly as symmetrical muscles ; 

 but the necessary movements of the two eyes require the 

 contraction of the superior oblique of one side, to be asso- 

 ciated with the contraction of the inferior oblique and the 

 relaxation of the superior oblique, of the opposite side. 

 For this, the fourth nerve of one side is made to .act with 

 a branch of the third nerve of the other ; as if thus the 

 tendency to simultaneous action through the similar nerves 

 of the two sides were prevented. At any rate, the rule of 

 distribution of nerves here seems to be, that when in 

 frequent and necessary movements any muscle has to act 

 with the antagonist of its fellow on the opposite side, it 

 and its fellow's antagonist are supplied from different 

 nerves. 



Physiology of the Fifth or Trigeminal Nerve. 



The fifth or trigeminal nerve resembles, as already 

 stated, the spinal nerves, in that its branches are derived 

 through two roots ; namely, the larger or sensitive, in 

 connection with which is the Gasserian ganglion, and the 

 smaller or motor root which has no ganglion, and which 

 passes under the ganglion of the sensitive root to join the 

 third branch or division which issues from it. The first and 

 second divisions of the nerve, which arise wholly from the 

 larger root, are purely sensitive. The third division being 

 joined, as before said, by the motor root of the nerve, is of 

 course both motor and sensitive. 



Through the branches of the greater or ganglionic por- 

 tion of the fifth nerve, all the anterior and antero-lateral 

 parts of the face and head, with the exception of the skin 

 of the parotid region (which derives branches from the 

 cervical spinal nerves), acquire common sensibility ; and 

 among these parts may be included the organs of special 



