346 



ANATOHY OF THE CENTRAL NERVOUS SYSTEM. 



newborn, therefore, in a cross-section of the cord the pyramidal tracts appear 

 gray among the white fibers surrounding. 



With the lower animals, corresponding to the more meagre expansion of the 



cerebral cortex, the pyramidal tracts are rela- 

 tively smaller than in man. Even here they 

 contain probably only fibers for those muscles, 

 which principally are used by the co-operation 

 of the cortex, — hence, in such actions as are 

 deliberate and acquired. At any rate, after 

 the separation of those of their fibers which 

 supply the upper extremities they are ma- 

 terially reduced in size, remain approximately 

 equal throughout the dorsal cord, and after 

 losing the fibers for the lower extremities they 

 are so reduced in volume that they are prac- 

 tically wanting in the lower lumbar cord. 

 Examinations of these tracts in animals that 

 make greater use of the hands — apes, burrow- 

 ers — and in those that use the hind-extremities 

 the more — as some marsupials — were desirable. 

 But they would have to be based on embryo- 

 logical or degenerative data, as only in such 

 wise can the pyramidal tracts be differen- 

 tiated. 



The examination of secondarily de- 

 generated spinal cords enables us to know 

 still more of the combinations of the 

 white columns. In a cord that has been 

 destroyed in the dorsal region by pressure 

 or some other lesion one finds, as the 

 foregoing leads one to expect, below the 

 place of lesion the corresponding pyra- 

 mid degenerated. But there appears also 

 a degeneration in the other direction. 

 It includes, near the lesion, the entire 

 area of the posterior columns; but, a few 

 segments higher, is confined to the me- 

 dian portion of these columns, that part 

 next the median fissure. In such sec- 

 tions we can easily distinguish, in the 

 posterior columns, an external and an 

 internal tract. That which degenerates 



Fig. 22.-Secondary descending de- ^^P^'^^'-^ ^^ ^^^^^^ columns (as far as 

 generation after lesion in the left t^G medulla oblongata) is fibers com- 

 cerebral hemisphere. (After Erb.) ing from the dorsal roots, and cut 



