SPINAL CORD AND COMMENCEMENT OF MEDULLA OBLONGATA. 189 



We left off studying the section of the oblongata at the 

 point where the central canal expands into the rhomboidal fossa. 

 The first nuclei of the cranial nerves had begun to appear 

 below this point. The fibres of the acoustic arise from cells in 

 the lateral portion of the anterior horn. From a point some- 

 what ventrad of the latter, perhaps representing the position of 

 the former anterior horn, arise the fasciculi of the hypoglossal 

 nerve. 



This is diagrammatically shown in the accompanying figure. 



FIG. 114. 



Section through the medulla oblongata at the level of the most posterior roots of the 



hypoglossal nerve. (Diagrammatic.) 



Aus H. Str. Kfrnr-n, From the nuclei of the posterior columns. An* R>i<-kenmark, From the spinal cord. 



Cereb. Jiahn, Lateral cerebellar tract. JBKnterhorn, Posterior horn. //'/<'< r.-;ii-iiit</? t Posterior columns. 



Schlei/e, Fillet. Vurder-Seitenhorn, Antero-lateral horn. 



If, looking at this diagram, you will remember that the central 

 canal broadens out along with the divergence of the posterior 

 columns and becomes the fourth ventricle, you will easily under- 

 stand that all nerve-nuclei developed in the floor of the ventricle 

 above this point must lie on the floor of the rhomboidal fossa. 

 The following cut (Fig. 115) shows this clearly. External to 

 the nerve-nuclei lie the posterior columns (now nearly devoid 

 of fibres), with their respective nuclei. The posterior horn, 

 recognizable by the substantia gelatinosa of its head, is entirely 



