()8 T11K XKHVOUS SYSTKM AM) ITS COXSKKYATION 



divisions in man is tin- same as in the frog. It is true in 

 both cases that the medulla is the prolongation of the cord 

 within the skull. In both, the fourth ventricle practically 

 breaks through the "roof" or dorsal surface of the medulla. 

 but in man it is overhung by the cerebellum. In both 

 types the aqueduct of Sylvius leads through the region 

 known as the midl train and widens into a third ventricle 

 reckoned as the cavity of the diencephalon. 



In the brains of the higher animals there is a ventral 

 feature hardly indicated in the frog, but developed, 

 generally speaking, in proportion to the prominence of the 

 cerebellum. This is the pons (pons varolii), a structure 

 which presents the appearance of an outgrowth from the 

 cerebellum clasping the medulla near its anterior border. 

 Of course, this does not correctly describe the actual con- 

 nections of the region referred to, though it is true that it 

 contains many transverse fibers which pass into the 

 cerebellum. It is to be noted that the cerebellum is dis- 

 tinctly dorsal in position to the main axis of the central 

 nervous system. It may thus be said not to lie upon the 

 direct course of the ascending and descending impulses 

 which enter and leave the cranial cavity. So it is possible. 

 for experimental purposes, to remove the cerebellum and 

 leave this chief highway still open for service. 



The removal of the cerebellum, with the object of ob- 

 serving the deficiencies which follow and so drawing in- 

 ferences in regard to its previous activity, is permissible. 

 Hut the physiologist cannot excise the midbrain or the 

 medulla for any such purpose, since he would lose not 

 only the influence of the part withdrawn, but would 

 check the passage of impulses to and from regions an- 

 terior to the seat of operation. The destruction of the 

 medulla would leave no line of Communication between 

 the cerebral hemisphere- and the trunk. 



A few words may now be said about the distribution 

 of gray and white matter in the central nervous system. 

 In the cord the gray matter is internal and appears in a 

 crossret inn in a characteristic form often called an 



