462 TEXT-BOOK OF PHYSIOLOGY. 



activity and in volitional control of its muscles. Its movements are 

 therefore largely, if not entirely, reflex in character. 



Embryologic and histologic examination of the spinal cord and 

 medulla show that so far as their mechanisms for independent phys- 

 iologic activities are concerned both are fully developed. Similar 

 investigations of the cerebral hemispheres and of the nerve-fibers 

 which bring their nerve-cells into relation with the spinal segments 

 show that the cells of the cortex are not only immature, but that their 

 descending axons are incompletely invested with myelin. With the 

 growth of the child, psychic life unfolds and volitional control of mus- 

 cles is acquired. Coincidently the cells of the cerebral cortex grow 

 and develop and the fibers become covered with myelin. 

 (. The nerve-fibers which have their origin in the cells of the cerebral 

 cortex, and which terminate in tufts around the cells in the anterior 

 horns of the gray matter of the spinal segments, are to be regarded as 

 long commissural tracts uniting and associating these two portions 

 "of the central nerve system. 



Experimental investigations and observations of pathologic lesions 

 accord with the view that physiologically these fibers are efferent 

 pathways for the transmission of motor or volitional impulses from 

 the cortex to the spinal segments. The nerve-cells in which the 

 motor impulses originate are located for the most part, as will be fully 

 stated later, in the central portion of the cortex of the cerebral hemi- 

 spheres in the neighborhood of the central or Rolandic fissure. ^The 

 axons of these cells from each hemisphere descend through the 

 corona radiata to and through the internal capsule, along the inferior 

 surface of the crura cerebri, behind the pons to the medulla, of which 

 they constitute the anterior pyramids. (Fig. 209.) At this point the 

 pyramidal tract* of each side divides into two portions, viz^" 



1. A large portion, containing from 80 to 90 per cent, of the fibers, 



which decussates at the lower border of the medulla and passes 

 downward in the posterior part of the lateral column of the 

 opposite side, constituting the crossed pyramidal tract; as it 

 descends it gradually diminishes in size as its fibers or their 

 collaterals enter the gray matter of each successive segment. 



2. A small portion, containing from 20 to 10 per cent, of the fibers, 



which does not decussate at the medulla but passes downward 

 on the inner side of the anterior column of the same side, con- 

 stituting the direct pyramidal tract or column of Tiirck. This 



* From the fact that the region included between the origin of these fibers and 

 the internal capsule presents somewhat the form of a pyramid with four sides, 

 Charcot designated it the pyramidal region and the fibers composing it the pyram- 

 idal tract. The base of the pyramid includes the cortex of the convolutions 

 around the Rolandic fissure. The summit of the pyramid is truncated and covers 

 the pyramidal region of the internal capsule. 



