THE NERVOUS SYSTEM. 587 



Distribution of the Gray Matter. 



(i.) In the bulb, at the lower part the distribution of gray matter fol- 

 lows that which prevails in the cord. Higher up the chief part is found 

 toward the posterior or dorsal aspect, surrounding the central canal. 

 When the central canal opens out into the fourth ventricle the gray 

 matter comes to that surface chiefly, and is found to consist more par- 

 ticularly, on either side, of the nuclei of origin of the cranial nerves, 

 viz., the 12th,. llth, 10th, 9th, and 8th, and more externally of the 

 nucleus gracilis and nucleus cuneatus (n.g., n.c., figs. 361, 362). In 

 addition to these masses of gray matter, there are the olivary bodies (o, 

 figs. 361, 362) toward the ventral surface with the accessory olives (0'), 

 and the external arcuate (n.ar. in figs.) nuclei, placed at the tip of the 

 anterior fissure on either side on the ventral surface of the anterior 

 pyramids. 



(ii.) Inthepons Varolii. In addition to the origins of nerves in 

 the floor of the fourth ventricle on the dorsal aspect of the pons, viz. , 

 of the 7th, 6th, and 5th nerves, there are several masses of gray matter, 

 viz., in the back part, the superior olive (fig. 362), and in the front 

 part the locus cceruleus, as well as small amounts of the same material 

 mixed with fibres in the more ventral surface. 



(iii.) In the mid-brain, the gray matter preponderates in the optic 

 thalami, corpora quadrigemina, and corpora geniculata. It is also found 

 surrounding the aqueduct of Sylvius, and in other parts of the crura, 

 notably such masses as the red nucleus (fig. 363), locus niger (fig. 365). 



(iv.) In the cerebral hemispheres, the cerebral cortex is made up of 

 gray matter which incloses white matter, and the corpus striatum is 

 made up more or less of the same material. 



(v.) In the cerebellum, the gray matter forms the incasing material. 

 In the interior too there are masses of gray matter forming the corpora 

 dentata. 



This then roughly indicates the localities in which gray matter is 

 found ; the arrangement of the fibres and their relationship to the gray 

 matter will be dealt with later on. 



THE BULB OR MEDULLA OBLONGATA. 



The medulla oblongata (figs. 358, 359), is a .column of gray and 

 white matter formed by the prolongation upward of the spinal cord and 

 connecting it with the brain. 



Structure. The gray substance which it contains is situated in the 

 interior and variously divided into masses and laminae by the white or 

 fibrous substance which is arranged partly in external columns, and 



