STRUCTURAL PLAN OF THE HUMAN BRAIN. 381 



layer of the spinal cord. The bundle of nerve fibers is known as 

 the solitary tract. Although the relations are complicated and 

 not easily rendered clear, I hope enough has been said to demon- 

 strate that the dorsal zone always remains what it is at first — the 

 zone into which the ganglionic fibers enter and in which they 

 chiefly ramify. 



As every one knows, the two largest divisions or parts of the 

 human brain are the cerebrum or hemispheres and the cerebel- 

 lum. These, we have now learned, are both structures developed 

 exclusively from the dorsal zones of His, and have therefore a 

 very different morphological value from what has hitherto been 

 assumed — not being modifications of the whole brain, but only 

 local developments of the dorsal half of the brain. Just as primi- 

 tively the medullary fibers which arise in the dorsal zone pass 

 into the ventral zone, so in the specialized cerebral hemispheres 

 and in the cerebellum there arise very numerous nerve fibers, 

 but these still obey the primal law and take their courses into the 

 portions of the brain representing the ventral zones, and thence 

 the fibers are distributed to their various destinations. Until the 

 relations of the zones to the nerve fibers, on the one hand, and to 

 the hemispheres and cerebellum on the other, had been embryo- 

 logically determined, it could not be known that the course of 

 the cerebral and cerebellar fibers is in accordance with a funda- 

 mental law of nervous organization. We can foresee, though 

 somewhat vaguely, that essential physiological deductions will 

 follow the application of the law to the study of the functions of 

 the brain. 



The relations of the zones in the entire brain are indicated by 

 the diagram on page 374, which scarcely calls for comment, 

 since it sufficiently explains itself. I need only add that the posi- 

 tion of the dividing line of the zones in the region of the corpora 

 quadrigemina is somewhat uncertain. In the embryo this region 

 is known as the mid-brain, and shows the primary division very 

 clearly ; but as the further development has not been worked out 

 properly yet, we can not decide positively as to the exact demar- 

 cation of the zones in the adult.* 



The ventral zone of the brain may be defined, as we have al- 

 ready learned, as the territory of the medullary fibers, for it fur- 

 nishes the pathway for those fibers to collect in bundles, which 

 may either form nerve roots (ventral or lateral), or may cross, as 

 so-called commissural fibers, from one side to the other in order to 

 establish the nervous connection between the two halves of the 



* I am led to suppose that the dorsal zones of the mid-brain unite, but that the ventral 

 zones do not, and that therefore the aqueduct of Sylvius lies entirely between the ventral 

 zones, the dorsal portion of the original cavity in that region of the brain being obliterated. 

 It is very possible that this supposition is incorrect. 



