536 



THE NERVOUS SYSTEM 



column of the cord are known as the direct or uncrossed pyramidal tract. 

 These two pyramidal strands of fibers are those which degenerate after lesions 

 of the parts of the cerebrum known as the motor areas of the cortex. They 

 can therefore be traced downward after such lesions as tracts of degeneration. 

 They are the descending fibers of communication between the cerebral motor 

 cells of the cortex and the different segments of the spinal cord. The outer 

 borders of the anterior pyramids of the bulb are marked by the exit from that 

 part of the nervous axis of the twelfth or hypoglossal nerve. Still more later- 

 ally than this nerve there is on either side a rounded elevation or column which 



Optic chiasma 

 Optic tract 



Corpus geniculatum 

 extern um 



Corpus geniculatum 



internum 



Locus perforatus 



posticus 



Middle peduncle 

 cf the cerebellum 



Restiform body 



Oli 



Pyramid 



Anterior superfici 

 arcuate fibres 



Decussation of. 

 pyramids 



Optic nerve 

 Infundibulum 

 Tuber cinereum 



rpora mammillaria 

 Oculomotor nerve (III.) 



Trochlear nerve (IV.) 

 winding round the crus 

 cerebri 



Trigeminal nerve (V.) 



Abducent nerve (VI.) 

 Facial nerve (VII.) 

 Auditory nerve (VIII.) 



Vago-glossopharyngoal 

 nerve (IX. and X.) 



Hypoglossal 

 nerve (XII.) 



Spinal accessory 

 nerve (XI.) 



First cervical nerve 



FIG. 376. Front View of the Medulla, Pons, and Mesencephalon of a Full-Term Human 



Fetus. (Cunningham.) 



is known as the olivary body. It begins at a level a little lower than the open- 

 ing 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 poste- 

 rior fissure is the posterior pyramid. 



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 cornua are pushed more to each side by the large number 

 of sensory fibers ascending in the posterior columns, and terminating in the 

 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. 



