374 ANATOMY OF THE CEXTKAL NEHTOUS SYSTEM. 



tended, the nervus vagus (X) and the nervus glosso-pharyngeus {IX) 

 have their exit from the medulla. Jnst behind the pons, and laterally, 

 are the origins of the nervus acuslicus (VIII) and the nervus facialis 

 (VII). The sixth cranial nerve, the ahducens, arises internally from the 

 two last named. From deep in the pontile fibers springs the trigeminus 

 (V). The origins of the trochlearis (IV) and the oculo-motorius {III) we 

 have already learned. The trochlear comes from the velum medullare an- 

 ticum behind the corpora quadrigemina; the motor oculi ventrally from 

 the pes cerebri. 



The last section of the medulla considered was at that level where the 

 central canal expands into the fourth ventricle. Even before this the first 

 nuclei of the cranial nerves have appeared in the gray matter surrounding 

 it. The fibers of the accessorius arise from cells in the most lateral portion 

 of the ventral horn, and from a place anterior to it, which corresponds to 

 the base of the former anterior horn, the nucleus hgpoglossi, the fibers of the 

 hypoglossal nerve develop. 



In the two accompanying figures this is represented. If it be noticed 

 how the central canal, following the separation of the posterior columns, 

 spreads out and becomes the fourth ventricle, it will then be easily under- 

 stood that from this point on all the nerve-nuclei must lie in the floor of 

 this ventricle. This is clearly shown in Fig. 239. Laterally from these 

 centers lie the posterior columns, now sparsely provided with fibers. The 

 posterior horn, discernible by the substantia gelatinosa of its head, is 

 entirely separated; and the basal part of the lateral horn, too, from which 

 come the fibers of the motor accessorius, loses its connection with the com- 

 pact gray matter a little higher than the plane of the figure. It continues 

 as a column rich in ganglion-cells ventral to the gray matter as far as the 

 pons, and, after the accessory has made its exit, gives out fibers to the vagus 

 (and glosso-pharyngeus?), which first ascend in a dorsal direction, and then 

 bend around to the respective nerve-trunk (motor vagus, etc., nucleus). 

 Higher still it is met as the facial center. It must, therefore, be remarked, 

 that, with the exceptions of the hypoglossus and the motor nerves to the 

 eye, all the motor fibers of the cranial nerves arise from a cellular column, 

 which lies in a prolongation upward of the lateral cells of the ventral horn. 



Next one sees, in Fig. 338, what has become of the remainder of this 

 horn, and how greatly the olivary bodies have enlarged. When the lateral 

 horn has become separated, there appears where the posterior horn was 

 inserted — in a place, therefore, which held nuclei of sensory nerves lower 

 in the cord — a new, large nerve-center, with spindle-shaped cells, very 

 similar to those of the dorsal horn, the sensory center of the vagus nerve. 

 It lies in the floor of the fourth ventricle, median to the ala cinerea (Fig. 

 236), and extends forward to about the middle of the white striae acusticEe. 



