428 THE NERVOUS SYSTEM. 



the third nerve of the other; as if thus the tendency to simul- 

 taneous 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 move- 

 ments 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 trigeininal 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 gan- 

 glion 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 sen- 

 sitive. 



Through the branches of the greater or ganglionic portion 

 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 in- 

 cluded the organs of special sense, from which common sensa- 

 tions are conveyed through the fifth nerve, and their peculiar 

 sensation through their several nerves of special sense. The 

 muscles, also, of the face and lower jaw acquire muscular sen- 

 sibility through the filaments of the ganglionic portion of the 

 fifth nerve distributed to them with their proper motor nerves. 



Through branches of the lesser or non-ganglionic portion of 

 the fifth, the muscles of mastication, namely, the temporal, 

 masseter, two pterygoid, anterior part of the digastric, and 

 mylo-hyoid, derive their motor nerves. The motor function 

 of these branches is proved by the violent contraction of all 

 the muscles of mastication in experimental irritation of the 

 third, or inferior maxillary, division of the nerve; by paralysis 

 of the same muscles, when it is divided or disorganized, or 

 from any reason deprived of power ; and by the retention of 

 the power of these muscles, when all those supplied by the 

 facial nerve lose their power through paralysis of that nerve. 

 The last instance proves best, that though the buccinator mus- 

 cle gives passage to, and receives some filaments from, a buccal 

 branch of the inferior division of the fifth nerve, yet it derives 



