Herrick, Subdivision of the Brain. ^gy 



the dorsal roots, probably by way of the rami communicantes of 

 the sympathetic system. 



3. Visceral, or splanchnic motor. The region of the inter- 

 mediate zone and columna lateralis grisea. The efferent fibers 

 are pre-ganglionic sympathetic nerves, which leave the central 

 nervous system mainly at least by the ventral roots and which 

 excite the intero-effectors, including viscero-motor, vaso-motor, 

 excito-glandular fibers, etc. 



4. Somatic motor. The columna ventralis grisea and asso- 

 ciated fiber paths. The efferent impulses go out by way of the 

 ventral roots to the extero-effectors (somatic or skeletal muscles). 



Parallel with this longitudinal differentiation of the neural tube 

 a transverse segmentation took place called forth by the metam- 

 erism of the body. In all vertebrates, especially in the spinal cord, 

 this transverse segmentation is much more evident anatomically 

 than the longitudinal divisions. Accordingly, morphologists in 

 the past have devoted their attention almost exclusively to it in 

 the elaboration of metameric schemata of nervous organization. 

 But in reality transverse segmentation is far less important to cere- 

 bral morphology save for convenience of anatomical description, 

 as will appear from a consideration of the genesis of the two types 

 of specialization in question. 



We cannot hope to elaborate a nomenclature reflecting perfectly 

 the relations in a low type of brain like the lamprey's which will at 

 the same time be adequate for the human neurologist; but we should 

 seek to devise a scheme which is sufficiently elastic to permit of 

 adaptation to both with no change of fundamental plan. A sys- 

 tem based on transverse segmentation, while in many respects 

 better adapted for the lowest vertebrates, breaks down completely 

 when applied in the Mammalia. 



Metamerism is primarily mesodermal in origin. It arose as an 

 aid to locomotion of the vermiform type in very primitive animals. 

 The segmentation of the skeletal, nervous and vascular systems, etc., 

 is all secondary to that of the body muscles, and where these latter 

 disappear, as in the head of higher vertebrates, the neuromeres 

 lose their individuality also. The longitudinal functional divi- 

 sions, on the other hand, are primarily nervous. They represent 

 the most fundamental factors in the architecture of the vertebrate 

 body and exert a more potent influence on cerebral structure the 

 higher we go in the evolutionary series. Metamerism is more 



