6o8 



THE NERVOUS SYSTEM 



ally there is on either side a rounded elevation or column, the olivary body. 

 It begins at a level a little lower than the opening of the fourth ventricle. On 

 the dorsal side of the olivary body is the line of origin of the eleventh, tenth, 

 and ninth nerves, and from this to the posterior fissure is the region corre- 

 sponding to the lateral and posterior columns of the cord. 



The changes in structure which are noticed in a series of sections of the 

 bulb from below upward may be summarized: In the dorsal or posterior re- 

 gion, the posterior columns are pushed more to each side by the large number 

 of sensory fibers ascending in the posterior funiculus and terminating in the 



Optic chiasma 



Optic 



Corpus geniculatum 

 extern um 



Corpus geniculatum 



internum 



Locus perforatus 



posticus 



Middle peduncle 

 of tbe cerebellum 



Restiform body 



Oli 



Pyramid 



Anterior superficia 

 arcuate fibres 



Decussation 

 pyramids 



Optic nerve 

 Infundibulum 

 Tuber cinereum 

 Corpora mammillaria 

 culo-motor nerve (III.) 



Trochlear nerve (IV.) 

 winding round the crus 

 cerebri 



Trigeminal nerve (V.) 



Abducent nerve (VI.) 

 Facial nerve (VII.) 

 Auditory nerve (VIII.) 



Vago-glossopharyngeal 

 nerve (IX. and X.) 



Hypoglossal 

 nerve (XII.) 



.Spinal accessory 

 nerve (XI.) 



First cervical nerve 



FIG. 378. Front View of the Medulla, Pons, and Mesencephalon of a Full-term Human 



Fetus. (Cunningham.) 



gracile and cuneate nuclei. The substance of Rolando is increased and be- 

 comes rounded, reaching almost to the surface of the bulb on each side, only 

 a small tract of longitudinal fibers of the root of the fifth nerve intervening. 

 There is a great increase of the reticular formation around the central canal. 

 Then at the ventral or anterior aspect the decussation of the pyramids begins. 

 By this crossing over of the fibers, the tip of the gray anterior cornu is cut off 

 from the rest of the gray matter. The central canal is pushed farther toward 

 the posterior surface, first of all by the decussation of the anterior pyramids 

 just mentioned, and later on, i.e., above, by another decussation of more 

 dorsal fibers. These fibers of the second decussation as they cross form a 

 median raphe and also help to break up the remaining gray matter into what 

 is called a reticular formation. These fibers arise from the nuclei of the funic- 



