THE CENTRAL NERVOUS SYSTEM 



721 



looked at, owing to the unopposed action of the inferior rectus 

 There is also diplopia on looking down. Unlike the other cranial 

 nerves, the two trochlear nerves decussate completely after they 

 emerge from their nuclei of origin. 



The fifth or trigeminus nerve appears on the surface of the pons 

 as a large sensory root and a smaller motor root. Its deep origin is 

 more extensive than that of any of the other cerebral nerves, stretching 

 as it does from the level of the anterior corpus quadrigeminum above 

 to the upper part of the spinal cord below. Its sensory root, in fact, 

 seems to include the sensory divisions of several motor cranial nerves. 



The motor root arises partly from a nucleus in the floor of the 

 fourth ventricle below the pons, partly as the so-called descending 



FIG. 259. SCHEME OF MOTOR AND SENSORY NEURONS OF TRIGEMINUS 

 (BARKER, AFTER GEHUCHTEN). 



G. s. G., Gasserian ganglion; Nu. m. m, n, V., nucleus of the descending root; 

 Nu. m. pr. n. V., chief motor nucleus of the fifth nerve; Rad. desc. mes. n. V. t 

 descending root; Tr. sp. n. V., tractus spinalis, or ascending root of the fifth. 



root from large nerve-cells scattered in the grey matter around the 

 aqueduct of Sylvius all the way from the anterior quadrigeminate 

 body to the point at which the motor root is given off. 



The sensory root has likewise two nuclei of reception : a nucleus in 

 the neighbourhood of the motor nucleus in the floor of the fourth 

 ventricle, and a long tract of scattered cells, the so-called spinal root, 

 extending upwards from the level of the second cervical nerve through 

 the medulla oblongata and the tegmentum of the pons, where it lies 

 external to the descending root. The sensory fibres of the fifth 

 have their cells of origin in the Gasserian ganglion. 



The motor fibres of the fifth nerve supply the muscles of mastica- 



