5 6o TEXT-BOOK OF PHYSIOLOGY 



control of muscles 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 hemispheres 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. 231). At this point 

 the pyramidal tract of each side divides into two portions, viz. : 



1. A large portion, containing from 85 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 15 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, constituting the direct 

 Pyramidal tract or column Ttirck. This tract can be traced down, as 

 a rule, only as far as the mid-dorsal region. As it descends it becomes 

 smaller as its fibers cross the anterior commissure to enter the gray 

 matter of the opposite side. Thus all the fibers of the pyramidal tract 

 from each cerebral hemisphere eventually are brought into relation with 

 the cells of the gray matter of the opposite side of the cord. 

 That the pyramidal tracts are the -conductors of volitional impulses 

 throughout the length of the cord to its various segments has been made 

 evident by the results of section, electric stimulation, and disease. Division 

 of the ventral and lateral funiculi of one side of the cord in any part of its 

 extent is invariably followed by a loss of motion or paralysis of the muscles 

 below the section, while electric stimulation of the peripheral end of the 

 isolated crossed pyramidal tract is followed by marked characteristic move- 

 ments of the muscles. Similar results follow division of the pyramidal tract 

 in any part of its course from the cerebral cortex downward. Electric 

 stimulation of the cortical cells which give origin to the pyramidal tract is 

 also followed by contraction of the muscles of the opposite side, while their 

 destruction is attended by paralysis of the same muscles. As the nutrition of 

 the fibers is governed by the cells, it follows that when the axon is separated 

 from its cell-body it degenerates. It has been found that a lesion of the 

 pyramidal tract in any part of its course is followed by descending degenera- 

 tion, which is taken in evidence that it conducts nerve impulses from above 

 downward. Thus experimental investigation and pathologic observation are 

 in accord in the view that physiologically these nerve-fibers are the pathways 



