CRANIAL NERVES. 631 



For the following brief account of the most recent views relating to the origin 

 of the cranial nerves, the editor is indebted to his friend Mr. Lockhart Clarke : 



The third cerebral nerve arises chiefly from two large masses of gray sub- 

 stance at the floor of the iter e tertio ad quartum ventriculum beneath the 

 corpora quadrigemina. 



The fourth arises from two nuclei at the floor of the iter e tertio ad quartum 

 ventriculum, and from the valve of Vieussens, where the opposite nerves decus- 

 sate each other. 



"The large roots of the fifth or trigeminal arise chiefly from the gray tubercles 

 of Rolando, or the upper expanded extremities of the posterior gray horns of 

 the spinal cord; the small or motor roots arise from two masses of large, multi- 

 polar cells situated each on the inner side, and close to the gray tubercle, and 

 intimately connected with it. 



The sixth nerve arises in common with the facial from the gray substance 

 of the fasciculus teres on the floor of the fourth ventricle. 



The facial nerve has two origins: 1. From the gray substance of the fasci- 

 culus teres on the floor of the fourth ventricle. 2. From the nucleus of the 

 motor root of the trigeminus ; between these two origins it forms a loop along 

 the floor of the ventricle. 



The auditory nerve has three origins: 1. From the superior vermiform 

 process of the cerebellum; 2 and 3. From the inner and outer auditory nuclei 

 formed chiefly by the gray substance of the posterior pyramid and restifbrm 

 body. 



The vagus and glossop-haryngeal nerves have each two origins: 1. From a 

 special nucleus in the floor of the fourth ventricle. 2. From the anterior or 

 motor part of the medulla. 



The spinal accessory nerve has three origins: 1. The lower roots from the 

 anterior gray horn of the spinal cord in common with the motor roots of the 

 cervical nerves. 2. From the gray nucleus of the hypoglossal nerve. 3. From 

 a special nucleus behind the central canal of the medulla oblongata. 



For further information on the origin of these nerves, and on the connection between their 

 several nuclei, see Mr. Lockhart Clarke's memoir " On the Intimate Structure of the Brain," 1st 

 and 2d Ser. Phil. Trans. 1858 and 1868. 



For fuller detail concerning the Cranial Nerves, the student may refer to F. Arnold's " Icones 

 Nervorum Capitis." 



