AREAS OF THE MEDULLA OBLONGATA 



885 



The Abducent Nerve Nucleus. The abducent nerve is a small motor nerve, 

 supplying the External rectus muscle of the eyeball. Its nucleus of origin 

 with large, multipolar cells, lies close to the median plane beneath the eminentia 

 teres. The axones from these cells pass ventrad through the tegmentum and 

 trapezium, and laterad of the pyramidal tract, to emerge in the.postpontile groove. 

 The nuclei are brought under the dominion of the cerebral cortex by pyramidal 

 fibres of the opposite side. They are likewise brought into intimate relation with 

 the trigeminal, acoustic, and opposite oculomotor nerve nuclei. 



Decussating fibres Direct fibres to Restiform Eff ^' e ^ fibl : es 

 to sup. olivary mp. olivary body. V**WM* 

 body of om,osite body of same I acmtwum. 



side. > side 



ke. FoHrtfc 



act 



Central acoustic 

 tract (lateral 



Tuber culum 

 acusticum. 



Accessory nucleus 

 of auditory nerve. 



Efferent fibres of 

 accessory nucleus. 



Superior olivary 

 body. 





Decussating fibres to sup. 

 olivary body of oppo- 

 site side. 



Trapezoid nucleus. 



Trapezoid body. 

 Pyramidal tract. 



FIG. 650. Terminal nuclei of the cochlear nerve, with their upper connections. (Schematic.) The vestib- 

 ular root with its terminal nuclei and thin efferent fibres have been suppressed. On the other hand, in order 

 not to obscure the trapezoid body, the efferent fibres of the terminal nuclei on the right side have been 

 resected in a considerable portion of their extent. The trapezoid body, therefore, shows only one-half of its 

 fibres viz., those which come from the left. (Testut.) 



The Trigeminal Nerve Nucleus. The trigeminal is relatively enormous and has 

 correspondingly extensive central connections, including nuclei in the mid-brain, 

 pars dorsalis pontis and oblongata, and spinal cord. It is a mixed sensomotor 

 nerve and the afferent and efferent divisions must be considered separately. 



1. Afferent Portion. The axones of the afferent or sensor root arise in the cells 

 of the large semilunar (Gasserian) ganglion. As in the dorsal roots of the spinal 

 nerves, these axones bifurcate, on entering the brain axis, into ascending and 

 descending rami. These terminate in a cephalic nuclear extension of the gela- 

 tinosi Rolandi of the cord; the ascending rami terminate in the so-called sensor 

 nucleus of the trigeminus, the descending rami in the nucleus of the spinal tract 

 of the trigeminus, which extends as far as the second cervical segment of the cord. 

 The sensor nucleus, at the level of the entrance of the nerve, is quite massive, 

 becoming attenuated cephalad. The spinal tract, in its descent, likewise decreases 

 rapidly as it gives off its terminal axones to the nucleus of the tract. The cells 

 of these terminal nuclei send out axones which cross the median plane, giving off 

 collaterals to the facial nucleus, to join the medial lemniscus to reach the thal- 

 amus, and, via thalamus, the somesthetic cerebral cortex. Other axones are 

 distributed (a) to the motor or efferent nucleus of the trigeminus and (6) to the 

 motor or efferent cranial nerve nuclei. 



2. Efferent Portion. The efferent or motor component of the trigeminal nerve 

 consists of axones arising from cells in two nidi: (a) the principal nucleus in the 





