THE BRAIN. 



485 



of the cord as the region of the medulla is reached, and the pos- 



terior cornua become more and more external, the whole mass of 



gray matter flattening out, until in the medulla it forms a layer 



the outer portions of which represent the posterior horns and the 



middle portions the anterior. The 



posterior columns separate in the 



medulla, the central canal coming 



to the surface posteriorly and end- 



ing in the fourth ventricle, the floor 



of which is the gray matter above 



referred to, which is, however, not 



limited to this site, but is pres- 



ent also about the aqueduct of 



Sylvius. From this gray matter 



arise all the cranial nerves except- 



ing the olfactory and optic. 



The medulla, like the cord, has 

 an anterior and a posterior median 

 fissure. At the lower part of the 

 anterior fissure are fibers that cross 

 from side to side, the decussation of 

 the anterior pyramids. The pos- 

 terior fissure of the cord widens 

 out and forms the fourth ventricle. 

 Some of the cranial nerves come 

 out from the medulla, and serve as 

 boundaries to describe the different 

 portions of the medulla. That por- 

 tion of white matter between the 

 anterior median fissure and the 

 roots of the hypoglossal nerve is 



the anterior pyramid. The lat- 

 prnl pommn is hpfwppii flip rnnt^ 

 OI the hypoglossal and those OI 



the glossopharyngeal, the pneu- 



/ P . , 



mogaStriC, and the Spinal acces- and 3, the inferior maxillary divi- 



SOry. At the upper portion the sions ; VI, the left abducens nerve; 



! . J i J v i ,1 i VII. VIII, the facial and auditory 



olivary body lies between the col- ner ; es . jx-xi, the glossopharyngeal, 



limn and the pyramid. The pOS- pneumogastric, and spinal accessory 



terior column is between the lat- ^^K^ci 



eral column or tract and the pOS- cervical nerve (Nancrede). 



terior median fissure. It is com- 

 posed of three smaller columns separated by shallow grooves, the 

 most external being the funiculus of Rolando, next the funiculus 

 cuneatus, and the most internal the funiculus gracilis, the first 

 two being joined in the upper part of the medulla to form the 

 restiform body. The outer portion of .the pyramid is derived 



FIG. 279. View, from below, of the 

 connection of the principal nerves 

 with the brain : I', the right olfactory 

 tract ; II, the left optic nerve ; II', the 

 right optic tract (the left tract is seen 



III, the left oculomotor nerve ; IV, the 

 trochlear; V,V, the large roots of the 

 trifacial nerves: + +, the lesser roots 

 (the + of the right side is placed on 

 the Gasserian ganglion) ; 1, the oph- 

 thalmic; 2, the superior maxillary; 



