NERVOUS SYSTEM OF A TWO-HEADED PIG EMBRYO 415 



IV. The intracranial fibers of the fourth nerve are obscured 

 by the dense cells of the mantle layer, and I was unable to trace 

 their decussation (fig. 8) . 



V. Within the marginal zone of the medulla on each normal 

 side the sensory root fibers of the trigeminus form a flat longi- 

 tudinal tract just beneath the surface, which descends as a 

 typical tractus spinalis of the fifth nerve (figs. 11 to 15). Neither 

 the motor nucleus nor the mesencephahc root fibers could be 

 identified with certainty. 



VI. The intracerebral course of the abducens fibers was not 



traceable. 



VII. The motor fibers of the facial nerve from the surface of 

 the brain across the pons, beneath the floor of the fourth ventricle 

 to the genu internum, were readily followed, but from here to 

 their nucleus of origin it was not possible to trace them with 

 certainty (figs. 11, 12). The fibers of the nervus intermedius, 

 or sensory seventh, penetrate the brain in very close relation to 

 the eighth nerve, but I did not succeed in tracing them with 

 certainty across the tractus spinahs of the fifth nerve into the 

 tractus sohtarius, which is conspicuous in sections of the medulla 

 (figs. 11, 12, 13). 



VIII. The course of the acoustic fibers within the bram 

 was equally indefinite. 



IX and X. The intracerebral connections of these nerves are 

 intimately associated with the tractus solitarius. Afferent gan- 

 ghonic fibers from both nerves penetrate the brain by a series 

 of rootlets along the lateral wall of the medulla and pierce the 

 mantle and marginal layers almost at right angles, some of the 

 fibers to enter the tractus solitarius, others their terminal nuclei, 

 which he between the solitarius and the central canal (figs. 



12, 13). 



XI. Filaments of the spinal accessory arise from cells in the 

 anterior column of gray in the upper cervical segments of the 

 spinal cord and emerge from the lateral surface of the cord and 

 medulla as the spinal root of the accessorius. 



XII. The fibers of the hypoglossus were not traceable from 

 the surface of the brain to their nucleus of origin. 



